Tweaky:

Its mathmatical in nature. If a greyscvale varyies subtlely , it can create such bands due to the math of the grey scale interpreter, the imapcts try to fill in a grey scale of the line, so if the next line is only differing in very small ways, the math involeved could simply shift a bit one way, if each line is the same variation, then bands can occur.. not much you can do about it..

Art