That's because they're different things in the fediverse.
Lemmy upvotes are mapped to ActivityPub upvotes
Kbin upvotes are mapped to ActivityPub boosts (basically a retweet in twitter language)
This also means that Kbin users have to be much more careful with what they "upvote" as that causes the post/comment to be shown to everyone that follows them in the wider fediverse. They are looking to change this if I recall correctly, and there's a lot of communication between the devs of Lemmy and Kbin on how to approach both this issue, and unifying their API system so apps can work for both.
jeena