Follower counts are actually incredibly difficult, if not impossible, to get completely accurate, because there is nothing saved to YOUR kind 3 when someone follows you. There can't be, since they don't have access to your key to sign for the update.

Instrlead,who is following you is saved to all of the kind 3 events of each individual npub that follows you, and stored on whichever relays they publish to. If a client isn't checking all the relays that could possibly have a kind 3 from some random npub that might be following you, they will all end up with different numbers of followers, based on which relays they ARE checking.

Checking loads of different relays and individual kind 3 events on each one of them is also very resource intensive, so most clients just check the most used relays and maybe a few others and call it good, which is why you see such a variance in numbers.