Perhaps they see in their daughter aspects of their own personality (or that of daughter's father) that they don't particularly like but won't acknowledge?
That's kind of similar to what my Dad said to me about my mum - he said that she resented my youth and even more so as it was something she couldn't take from me. It did make me wonder that if he was that aware of what she was doing, why didn't he try to put a stop to it?
Then again, she could have a very different reason - as she has told me, I ruined her life the day I was born!
My Mum only liked her sons
Same here - my brother can do no wrong. When he was over from Australia visiting, he went for a walk in the local park. My mother came into the room and asked where he was and my daughter said 'he's gone for a walk on the lake' - naughty I know but I did chuckle (but kept the chuckle hidden).