Federation can get messy when you start to compare instances between each other.
For one some instances disable down votes, so may count differently.
Then you might see delays in synchronisation for whatever reason.
Finally, federation & defederation. suppose you have instance A, B, C, and D but instance A has blocked C. Users on C comment in and upvote content on D. Instances A&B will download a copy of the stuff posted by D users, but only B will have upvotes contributed by C. Therefore, different upvotes counts.