(Disclaimer: looks aren't important, blah, blah, blah...)
You meet someone for the first time. You get to know them as an individual - outside their regular life. You don't know their friends.
And when you meet them for the first time - you're shocked. Because...well, they don't look like each other.
You see, some girls have decided to be the hot fish in an ugly pond. Rather than struggle with evolving and uncertain social roles, they have chosen to align within a group where there is a clear alpha female - themselves. Everyone is more comfortable with this. The hot fish likes the attention. And the ugly fish can delude themselves into thinking they're part of the hot group, while never really fighting for the alpha female role.
This grouping may be subconscious. It may be intentional. But now, you're going to start noticing it everywhere.