Some of this may be a bit in the weeds for some readers, but this is a semi-technical introduction to how retargeting works, so that you can compose your retargeting triggers as accurately as possible to get what you are expecting.
This is a common concept in CRM's, marketing automation tools, as well as analytics and advertising tools. Briefly, a static audience is like a list of people which can only be changed by explicitly adding a person or removing them. This contrasts with a dynamic audience which is defined by a rule that can be used to evaluate a person, and if that person matches the rule, they are in the audience, otherwise, they are not.
There is no direct explicit way to add an individual to a dynamic audience without them meeting the rule for the audience. Similarly, there is no way to automatically and implicitly add people to a static audience without enumerating each individual who belongs in the audience.
To bring this to more concrete terms, a static audience may be a list of people's emails. You can only add or remove a specific email to the audience. A static audience is what you see in Woopra's segments or labels. You can define a number of rules that together make a dynamic audience and each time you ask the system who is in that audience, it will run the query and find exactly who is in it now. A dynamic audience will change over time as well even if you never alter its definition.
In order for two services like Woopra and AdWords to exchange any information about a person, there must be some sort of shared way of identifying that person so that the person is uniquely and unequivocally identified in both services. In this case, there isn't much information "about" the person that is being shared, simply the fact that you want this person to be in an audience in your advertising platform for specific messaging, etc., but the concept still applies.
Generally, services like Adwords and Woopra try to minimize the personal information that is shared to other services for privacy concerns among other reasons. Even if this weren't the case, often the people to whom you want to retarget are not even identified in your platform yet. While we can assume that Google and Facebook probably know who they are, there is no way that those companies will be sharing that information.
So how does it work?
Everyone has heard of cookies, but what are they??? Cookies are a vital part of remarketing, so a basic understanding of them will go a long way. The concept is simple but broadly used for different purposes, so it can seem daunting. Very simply, a cookie is a piece of information (usually just a random string) that uniquely identifies a browser to a server. This way, when a browser sends two requests to a server, that server can have the option of using cookies to know that both requests came from the same browser. A request is simply the browser asking a server for a web page.
Cookies are set on the browser by the server, and are only sent back to the domain that set them. So if you make a request to woopra.com, and then you login, Woopra will set a cookie on your browser that says you are logged in for future requests. This cookie could automatically become invalid if the server decides that you must log in, say, every 12 hours at least. It is an important security feature that is implemented in all modern browsers, that a cookie is only sent back to the server/domain that set it, and no other services can get access to it.
Advertisement platforms use this to be able to tell if a person who is not identified, and/or not logged in, is the same person between sessions, and even requests. This seems like a small thing, but it is a keystone in both tools like web analytics and remarketing, as well as in the very experience we have on the internet--it allows web services to provide a more personalized experience, remembering preferences, and showing us the content we tend to find, even if they don't know who we are.
So in short, a cookie is an anonymous identifier. Services use it to uniquely refer to a single person, even if they don't know who that person is and have no personal information on them. This is key to remarketing to people you don't yet know and have not signed up for your service.
One common approach is to do some sort of id swap. In this case, one tool will keep a database that maps their own ids to some external ids. Woopra does this with some other platforms so that when we want to refer to a person in another platform via an api call, we have that person's id in the other platform available to us. This of course requires that you have an identifier. But what do we do in this case when we don't know the visitor on our website, but we want to tell google to advertise to them? We use what I call, identification by ostension.
Ostension is definition by pointing. It is how one learns the names of colors, and a good deal of other things as young children. In the case of identifying people, what this means is that we get the AdWords code, the Woopra code, and the website visitor all in one place (on your site), then Woopra can tell AdWords, without using any identifiers, that we want to remarket to this person.
Often, retargeting code has to run on the browser because Woopra is using Identification by ostension to tell AdWords, etc. that you want this person added to a given audience. Since Woopra often does not even have an identifier for a user, and is not going to be able to get access to google's or facebook's id for the user, this is the only way to do it, as discussed above.
One major thing this means, though, is that you need the triggering action to be an event that is tracked from the browser. Woopra's clientside (browser-side) triggers can only be fired as the result of a client-side event.
The AdWords app in Woopra makes use of dynamic audiences only, and uses a cookie-based, clientside identification by ostension.
The AdWords app actually syncs all of your label data to AdWords and creates audiences in AdWords to sync with the Woopra labels you have selected.
The Facebook app allows the use of both cookie-based dynamic audiences, and email-based static audiences.
The ability to use email addresses and static audiences mean that you can add people to remarketing lists without having them on your site at the time you decide to add them. See Serverside vs. Clientside Triggers for details.
There are a number of reasons why you may find that your audience or segment in Woopra has more users in it than are reported in the matching audience in your remarketing tool.
The biggest reason this will happen is that sometimes, the remarketing tool does not know who the user is. This can especially be the case when creating static audiences based on email addresses. This can be done in Facebook Ads, for example, and where it goes wrong is if Facebook does not have a user associated with the email you've added to the list. One common reason this happens is that the user shared a professional or work email whereas they have signed up to Facebook using a personal email address.
Another reason that this happens is that data from many retargeting services is sampled. That means that you will only get an approximate number of people in an audience. Further, if the audience is growing quickly, it is likely that this approximate count in the remarketing audience is lagging by a couple days or even by a week behind the actual numbers of incoming people who are being added to the audience.