I can't improve much on that last post by Andrew. But it seems like we're focusing on a tree and calling it the forest. I'm willing to bet you have never seen the same match-up twice. The same song several times? Sure. In a category of 500 songs, you will eventually see it 500 times. But everyone else is experiencing an entirely different shuffle than the one you see, and I believe the balance comes out in the whole of the process.

http://www.soundclick.com/MarkKaufman