Curious about this too, using Damus. Was pretty accurate till recently. My high score went down by 1/3.
The follower number displayed on my app is 2/3 that displayed on https://nostr.band/
Does this mean 1/3 of people generally don't see my posts even tho they don't follow me?
Discussion
It just takes a long time to pull it all in from relays, the more followers the worse it gets. Nostr. Band and primal have it cached, so they keep the "real" number.