Okay, so think of every website that is part of the “Fediverse” (aka uses “ActivityPub”) as just being different ways to display the exact same data. Sometimes their data works really well between two of them and sometimes it’s a bit more awkward. Lemmy and Kbin are both trying to imitate the “forum-style” UI that Reddit uses, so they integrate really well with each other. Same data, slightly different UI. Mastodon, on the other hand is imitating Twitter. So trying to read Mastodon in Lemmy is like trying to read a Twitter feed as Reddit threads. It’s messier. Kbin seems to be trying to find a way to better display Mastodon-style threads within their UI. Otherwise, I think the big picture way to understand the difference is just that it’s a matter of UI and which one you prefer more.