That ‘all’ is all of the communities and posts your server knows about. You are on a pretty big (I think?) server, so it’s probably pretty good. For people on smaller servers like the one I’m on, it won’t have a lot of the smaller niche communities on there as no one from my server has ever visited them.
If I made a new community on my instance and posted stuff there, you wouldn’t see it in your ‘all’ feed unless someone from lemme.ee visited the new community first.