Right, but of course if you don't subscribe to it (and nobody else does) then it doesn't.
So, for instance, if you go in through an account on a narrowly specialized instance, you're potentially not going to see a lot of the communities from other instances at all, even on their All, just because nobody's bothered to subscribe to them. And you'll likely see highly specialized communities that fit well with that instance that you might not see anywhere else.
The smaller the instance is, the more likely that is.
I have accounts on a couple of small instances on which I haven't even bothered to subscribe to anything, since their All already matches what I want frim the instsnce.