Remember, all private schools are at root businesses. There are many institutional needs that must be satisfied when choosing a class.
One important need is that the more average academic kids not feel outclassed by too large a cohort of academic top achievers. There is only so much room for kids who are going to “make it look easy.”
Obviously, there are many more institutional needs that must be satisfied and which help explain why academic and even extracurricular achievement are no guarantees of admission.