I read somewhere (can look up) that black men's penises tend to have less size difference when they are erect than whites -- i.e. they average out to be about the same when erect, but when flaccid (I know you guys hate that word), black men's tend to be a bit longer. So in locker rooms or whatever, checking out happens and incorrect assumptions are made.
Kind of the same idea as noses -- caucasian noses evolved to be longer and narrower to deal with cold temps. And their penises evolved to get shorter and closer to the body to deal with same.
I have no idea where I got this stuff, will go try to find out if there is any veracity in a bit.