Why can't I see all comments on a post? - Lemmy.world

I posted a comment on this post: https://lemmy.world/post/270586 [https://lemmy.world/post/270586] My comment is the only one so far… I accidentally found that this same post had lots of comments if I access it via this other link: https://lemmy.one/post/190223 [https://lemmy.one/post/190223] Are these completely different posts because they’re in different Lemmy instances? Why don’t the two get “joined” together? Do they look separate because the user just posted them twice in different instances, or there’s some “per instance” comments going on?