The "Jelly Donut" Model
Americans thrive on choices.
I once read that immigrants from former Soviet countries were often overwhelmed by the selections they face in the supermarket; how many brands of bread, of toothpaste, of laundry soap, for heaven's sake?! Even meat comes in brands! Variety -- a/k/a diversity -- is the American way.
So why should an admissions officer be any different? Faced with hundreds of similar-looking applicants, the decision-maker may very well gravitate towards the more colorful ones (no pun intended). They won't take an inferior person just because they're different, any more than you'd take a squashed-looking chocolate cruller just because it was chocolate. But they will look for people who stand out -- and those people are often minorities.
How often? Take a look at this graph of applicants in 2005:
That blue line at the top represents applicants who identified themselves as white, and the second-highest line represents the applicants who didn't specify; all those tiny little bands at the bottom are the different minority groups. All minorities combined total just over 25,000, while Caucasians plus unidentified and unknown (the two blue lines) total about 65,000. If you were standing at that table full of donuts, which would you take?