First of all, you should be aware that the data you see on Meta, i.e. the number of followers and the number of likes, is not the same data as that displayed on your SOPRISM dashboard.
In Meta, the data displayed is the number of followers or likes of your fan page.
In SOPRISM, the data displayed corresponds to an estimate of the audience size. This data is used to show you the size of your audience.
The estimated audience size takes into account the active users, whereas the number of fans/likes corresponds to all the fans who at a given moment have liked the page without any notion of time.
👉 Knowing that the audience size depends on the people who can be reached by the ad, the estimated audience size will directly exclude people who have chosen not to see the page's publications anymore.
Finally, remember that being a fan does not necessarily mean that you interact with the page; in fact, you can like a page without seeing or interacting with its content. In this case, as for the first point, Facebook no longer counts people as active on the page.