I went to a similar school. It depended on major. If it was a major that the school was known for, say the top ranked CS school...then it was 100% that a grad was someone who was noticeably much, much smarter than most people. If they were at the same school but in a much less well recognized major (at my school...let's say history), then they were much closer to the average. If you are seeing students at Cambridge who are in prestige majors but without obvious skills...something else is happening and it isn't good. Otherwise, you are probably just seeing the differences between different majors.