Basically I made a post on blahaj and someone from kbin commented on it somehow even though we're not federated with them; I only happened to see the comment because I decided to look at how big the emojis looked to blahaj users so I went over to the same post on blahaj and there it was
When I first joined this was a common complaint for posts that were made before a community defederated. Basically a post and its comments are copied to each federated instance and then synchronized as long as the instances remain federated. kbin users are commenting on kbin's copy of the post which is then synchronized to any server that is federated with them.
We've actually never been federated with kbin though, we have a whitelist for who we federate with rather than a blacklist of who we defederate and kbin has never been on that list so how can kbin have a copy of my post?
Your post is on blahaj. kbin (or any federated instance) copied a version to their server so their users will see it in their "everything" feed. Kbin user comments on the post. kbin syncs that comment with all federated servers. Kbin isn't federated with hex so that comment doesn't get sent here and you won't see it on hex's copy of your post.
Ok thanks, that explains it <3