Mudd and colleges like it still end up with a range of student preparation in math and physics, despite only taking students who’ve had calculus and did really well as far as math grades and scores. The SAT I and II math tests and AP Calc tests don’t distinguish well in the 95-99.9% range.
There aren’t any standardized math tests to gauge ability at proofs, which becomes important at colleges like HMC. Even the AMC/AIME doesn’t do that. So, colleges like HMC and Caltech find out where to place students via placement tests over the summer.
Caltech does have a support track for some math and physics students that is below the regular track based on placement tests. The other freshman supports at Caltech seem similar to those at Mudd.
Note that for undergrads in recent years, HMC is about 46-47% female and Caltech is about 45-46% female.