They want you to dress nice (go figure what they mean by that), but if your clothes are better than theirs, they definitely won't like it.
It's like a woman criticizing a man because he's really in shape. "Wow, what a gym rat! and blah blah blah". Makes her feel better about herself and less guilty about how she let herself go.
Women are in competition with other women and the men they see. They'll feel insecure if you dress really well.
That's from my experience 8-)