Not completely certain, but my understanding is that follower count is calculated on the client, based on the information it has at that moment.

It can only count followers for which it has "Contact" information (kind 3), which it will gradually fetch.

For performance reasons, clients might cache this information and perhaps not correctly update this cache in the profile.

When looking at my profile, the follower count also changes based on how long I have my client open.

Still learning though, doing my own research ;-)

https://nostrdebug.com was certainly useful for this.

If anyone has other pointers for digging deeper, please let me know.