Having worked at Intel from June through December, I actually asked someone this question. I was told that they were supposed to be named after rivers. A river that flows through Portland, OR (near where I worked) is called Willamette. I thought it was pretty cool, but the new ones are not named after rivers. The guy I asked said he doesn't know what the hell they are doing now.
So, the answer to your question is that there used to be a system and now they are a bunch of retards. I can't speak for AMD though.