@lom I asked this question a few days ago in @kbinmeta . I think it’s to do with how upvotes (don’t) federate or how many comments are posted/ federate to an instance’s copy of a post.

The “All” that’s displayed on a given instance is all of the threads from that instance and from communities on federated instances to which someone from that instance has subscribed.

Roughly:

Imagine a brand new instance. For simplicity’s sake, we’ll imagine that it has no communities of its own, so the only content it can ever have is from other instances.

By default, it’s federated with everyone, but it’s not going to actually get anything from any of those other instances. It’ll just remain that way - federated but empty - until somebody from that instance subscribes to a community on another instance.

At that point, a mirror for that community will be created on the new instance. And from then on, that mirror will… well… mirror the threads and posts on the other instance.

But that’s it. At that point, the new instance has just gone from having no content to having content from one community on one other instance. And it’ll stay that way until someone subscribes to a second community, and then a third, and so on.

So again, “All” isn’t everything on the fediverse. It’s everything for which that specific instance has a mirror, which is everything the users of that instance have subscribed to.

So already to some degree, and undoubtedly more over time, instances are going to come to have a specific feel, as, for example, users on a tech-oriented instance subscribe to every tech community out there and not much of anything else, and so on.