Grades in high school are already like this. To get the best grade from a subject, you need to be good, but not exceptional. So being good but not exceptional in everything brings you the highest possible score.

If instead you are exceptional in a subject or two, and average in a few unrelated subjects, it gives you lower total score, and if the university cared about your grade average, you would have problems getting there, especially if many people with the highest total score competed with you.