For many people, finishing in the top of your class in highschool is challenging if you are capable but have had a relaitively difficult home life -- it requires broad spectrum high performance across years. Getting a good score on a single test is more achievable for some.

Now imagine the situation for a student with a poor family who finds themselves in a school mostly with students from wealthy families? The "top-%" metric would doubly harm them.

On the plus side, at least the top-% metric provides an incentive for students to attend 'worse' schools, which to the extent that students are benefited by mixing with more capable students, could help everyone.