Posts/comment from other instances aren't showing up when viewed from Lemmy.world.

I found this community on browse.feddit.de and wanted to join it. When viewing the community from that link, I see dozens of posts. But when viewing it from lemmy.universe, I only see two.

I've seen a few different instances of this. For example, this post has 2 comments on lemmy.ml, but 1 comment when viewing through lemmy.world.

I'm very new to Lemmy and the Fediverse. What's the issue here? Is this an issue with lemmy.world in particular, or Lemmy as a whole?