It's not just women, it's part of human nature.

Let's say you meet someone that you think is really cool. Before you know it, you'll probably start listing off things you have in common, and qualifying yourself so that they'll like you more.

In this case, if the girl thinks you're a high quality man, they will qualify themself to you to make themselves seem like a more attractive option. And that's because high quality men have their pick of the litter.

Because they want to stand out in the competition. If no competition is started, they'd not compete