Could be that blocked servers aren't feeding their votes back, but also lemmy instances (and fediverse platforms in general) don't sync data directly. Instead they all store local copies of posts and comments from various instances.

So when you upvote a post on an instance that isn't your home instance, your home instance updates the vote count for the post in its own database, not in the database of the instance the post is on.

For your vote to be counted on the other instance, your instance has to syncronize its database with the other one. And this is true for all instances. As such, vote counts on a post can be inaccurate across various lemmy instances depending on how recently any given instance has syncronized its database with the rest of the instances.