I can't offer much of an explanation, but I disagree with the subject area and "most qualified" explanations. Applications are forwarded by universities, and subject matter experts review them at the national level, and give them a score. Then as far as I know the highest scores across all subject areas get the awards... I don't think there's a quota for each subject area. And since person who posted said it was a pgs (not cgs) the most-qualified explanation doesn't seem correct.