This is not something I’ve encountered before, and there shouldn’t be anything that causes different OAuth tokens to get different results as it’s public data on that endpoint.
If the difference is just a small number, it could be due to caching. If you want to be certain you could use each OAuth token and page through all of the results to get 2 lists of followers and then compare them to see if some users that are in one are missing from the other, and compare the count of that list of users to the total being given. If there are users missing in one list, you could then use both from_id and to_id to see if they show up when specifically queried.