
It's the ones who are cute, but certain guys find unattractive.
It makes sense. From personal experience, I rarely try to chat up the most gorgeous girl at a party. Not that I haven't gone out with some extremely beautiful women, but as a guy on the prowl, I know how important it is to punch your weight. I therefore usually wind up talking to the "undervalued" females (as Popper puts it). These are people I know are cute and attractive, but I also recognize are not univerally seen as such.
Which, ironically enough, makes me think that this provides schlubs on the prowl with an opening: Maybe you should talk to the most beautiful girl in the bar if all her less-than-stunning friends are being monopolized.