Please explain. How can a song have a rating if I never rated it?
I've seen them come from Amazon's MP3 store with Ratings in the ID3 tags, though usually the source is... Less than legitimate. MC reads ratings in a variety of tagging formats, however, not just those used by iTunes (which does not use the "standard" ID3 Rating field for ratings tags -- the standard is something of a mess).
So, it is possible that file was tagged with a Rating in
another application (not iTunes, in a format iTunes doesn't use) and those ratings are coming through. iTunes wouldn't show them because it doesn't read them, but a full-fledged tagging application like MP3Tag would almost certainly.
If you don't ever want it to happen, add a
Tag On Import rule to always clear the [Rating] field.