I'm no expert in how high-end models are selected (or most kinds of models, for that part... I haven't cast a shoot in years (and that makes it sound more interesting than it was... haha)) but my understanding is that photographers want to work with interesting models. I guess if you get to a certain level, every woman is going to be at least arguably beautiful, and so whether she's a 9.75 or a 9.9 gets to be subjective... but if she has an incredible and/or incredibly different "look", then she's more valuable.
Huge eyes. Massive lips. Ethnic ambiguity. Crazy hair.
Those things don't make a girl more beautiful, necessarily, but they often make for a more interesting shoot and end product.
Ed O.