Goes without saying that the great tuba player also has to have great stats. But given the requisite great stats, that are almost as necessary as submitting an application, I believe that they are taking the kids with national level achievement in a specific field of interest, rather than the kids who have great stats and broad achievement confined to their high school - such as heading a couple of clubs and co-captaining a sports team. Even the kid with perfect grades, perfect scores, and head of student council and captain of a sports team is not necessarily getting in.
How many “Ask me how I did it” threads do you see where the theme is, “I have no idea why they took me at HYPSM other than the many, many others with similar achievements to mine.” Most of the people who were admitted have excellent stats plus either a hook or a spike.