If the median is A-, that means more than half the students are not getting an A. If we accept that Harvard does have some of the best students, well at least best at getting good grades, since that is at least partially how they get in, why shouldn't they have better grades on average? To do otherwise would seem unfair to the students, since people are often judged on grades by people who don't have any perspective as to what a given grade means at a given institution. It would not be surprising if getting an A at Harvard is still substantially harder on an absolute basis than getting an A at a much lesser school that has a much lower average.