Really Confusing Syndication

Lately I've noticed that some RSS feeds I have on Google Reader that I thought were just not updating actually are. I'm not getting notified that they're updating. What's up with that? Isn't the point of RSS to NOT have to go to the site to see if there's anything new? I'm thinking it's probably not a problem with the reader, but with the feeds themselves.

In any case, I have some catching up to do. I'll also have to check some of the feeds that are sitting empty and make sure they're not in limbo either.