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.
👉 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.