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.
Ok so in my case from your possible reasons it can only be a delay in synchronisation because dataterm.digital is not disabling downvotes and is also not defederated from lemmy.world and vice versa.
The upvotes could come from more than just those two places. If lemmy.world and dataterm.digital don't have identical global instance blocklists, one may still be blocking more upvotes counts han the other. Hence why my example had more than two instances.
Seems to me that the votes only count for the instance you're viewing from. Same with community subscribers, you can have 50 subs on one instance and 0 on another (for the same comm).