I think that much more likely plan-Bs (certainly career-wise) include studying informatics, information-system, industrial engineering, or simply computer-science in a less prestigious college. I doubt there are many who study math for any other reason than the love of it (at least not for their first degree).

Besides, girls do as-good (if not better) than boys in school, aren't they? So how can grades be an explanatory factor to begin with?