Ya, thats adsense for you, best ads go on the main search, leftovers get flipped to the other nets, pretty sure. Probably a matter of ads available vs traffic. Fluctuates a lot so is hard to know what to make of it. Best so far is about two and a half on one hit. Doesn't seem to be any correlation between traffic/hits and payout. More regularly updated sites seem to do better. Wouldn't swear on that, but data seems to trend that way.