Thanks stephaniel, that seems to be part of the reason but I notice it happens a lot even when questions aren't rated. Also, when you look at all the unrated answers below the rated ones, they're jumbled up.
Hey stephaniel, look at what I just did when I rated your answer! (thanks for same, btw) As you say, it's a mystery - looks like I'll just have to learn to live with it. Oh, well - such is life.