I think it's feasible that the females were significantly more qualified than the males. The fact that going to Caltech is seemingly against the gender norm for women may actually cause them to self-select for only the most qualified and dedicated to STEM to apply. Whereas a lot more men may have applied 'just because'.

Also, women are just doing better in school right now. The average female college applicant is better than the average male college applicant. So it's not hard to imagine this it true for CalTech too.