As a girls coach...and having asked, they roll them down because they ride up. I can tell you that we tend to watch the girls run, and know that by the second turn on the track, they will all yank their shorts down because they ride up...it is pretty funny. So they roll the shorts down so that they don't ride up. Girls have different 'equipment' and it is uncomfortable when the shorts ride up.
I think that clothing styles are different, and I also think that at many schools,the uniforms are a piece of crap and not well made, so that is why the shorts are rolled down.
During practice they may roll to get a tan or look good,but in meets it is a comfort issue.