>So, it's not "we have 100 slots this year, the 100 best get in"?
If a university's mission goes beyond just collecting already-top-performing students from the best (usually wealthiest) school districts, you can't just do a straight rank on a single signal. There's a lot of variance in educational opportunity across the US, ranging from highly selective specialty public high schools [1], to places where they can only afford 4 days of schooling a week [2].