I think women wearing heels and skirts look better. Legs are a great attribute... we won't get into peep toe shoes... ;)
Jeans and flats are a little more mundane, even if you look great in jeans. Do you really think that the same number of guys noticed you? Perhaps...
But while it might be that girls wearing skirts are more confident (because they are showing legs, for instance?), I don't think it means that she wants guys to notice her, particularly... she's just confident about herself and her femininity.
Normally a girl that's dressed up makes guys feel nervous. It frightens the majority of guys who lack confidence or guys who wants to stay away from the (generalisation) high maintenance a classy girl brings.
However you'd attracts guys who are confident. For some, really just want a girl to dress up so he can walk in a room and be treated like god by other men.
We definitely know consciously or un-consciously that a woman dressed up to get attention, we're not idiots, well most of us aren't..
Essentially, you're getting hit on less because you're not dressing up anymore, we see a hot looking woman who's dressed up so, we assume that she wants us to approach her, she's "presenting herself". Where as the opposite, she's dressing down, she's being conservative, she's doesn't want a guy to stare/hit on her. We'll be respectful and obliged accordingly.
casual outfits might seem too superficial. it would mostly make us guys think "whoa! she's way out of my league! no way I have chance with her, etc". I guess if your gonna dress up like that you might need to express yourself out a little more (long story)