Nostr accounts are trivially easy to create in any number, thus "followers" are arbitrarily many. Also to see the followers of Alice you need to download the follow list of all other users. A follower is an implicit property. Following is explicit. So you always get a different set of followers depending on the relays you connect to and the client you are using. Some clients consider only certain accounts and other clients might compute that number on the server and not update it every time you hit refresh.