How does the Subscribed feed actually work?

I've been using the Subscribed feed as my default view for a while. I understand that this is exclusively content from communities I've subscribed to, but it also seems to be be some subset of that content. If I go into an individual subscribed community, I almost always see a bunch of posts that I don't see on the Subscribed feed at all.

How does Subscribed choose which posts to show and how to order them?