Apps scan relays and sum the result, different data from different relays can lead to different results
Discussion
So who has the most accurate count? or all is accurate?
There is no 1 true count, some count all, some apply a spam filter. nostr.band is pretty well connected you can check that: https://nostr.band/?q=npub1h88z7vfm7mnaz94gn2p2a5psaduzkphrf2pndtxa4xvsd6zpzg8qajhqta