So, generally it is because you update who you are following from a different client, and every time you add a new follower, the client SHOULD be taking your existing follows to include them in the new follow list, and just adding the new follow to that list, too. This is because relays only keep the most recent version of your follow list and discard the rest.

If a client instead creates a new follow list with just the person you recently tapped "follow" on it, then the relays will discard the previous list they had, replacing it with the new list with only 1 follow.