Maybe other sites linking to yours aren't too picky with what terms they plaster THEIR sites with..?
I mean, if many such sites link to yours.. it's definitely at least noted by Google's
algorithms. I think it "rubs off" on your site... And that rubs off on AdSense.
So research who's linking to You, my guess is that your answer lies there..
You may unknowingly have scummy russian TGP's linking to You
