@sernoo, backlinks definitely don't account for every case. And while text changes don't take long to update in the SERPs, I've found Google often kind of sandboxes altered pages for a while before deciding just what to do with them. I wouldn't expect to see improved ranking from our on-page changes after the first refresh.
What I'm seeing is a different batch of search phrases every day, as if Google is testing each category of my site to see which one pleases visitors the most. Whether this even has anything to do with Penguin or not (or Panda, or just normal algo stuff), I have no idea.