Instance A doesn't know about a community on instance B until somebody on instance A searches for it and (I think) subscribes to it. After that, the entire instance A can see the community on instance B.

So, with small instances and new communities there's a delay until and unless somebody does a search for it and starts interacting. And that means that the 'All' on difference instances, especially smaller instances, is going to be different. As more and more users join various instances and search for communities and interact with communities in the rest of the fediverse these will be more synced.

Federation doesn't happen immediately, the syncs could be happening at a little bit different times across the different instances. Not enough to make any major differences in the content you're seeing or interacting with, but probably enough to make the 1:1 comparison not line up.

Each instance generates it based off of the subscriptions and the synced content that exists locally on the server. I think anyway...