May that be because tech is perceived as more competitive or stressful? Lots of people make fortunes these days doing technology and the pace is tremendous, unlike in sciences. Anyway, I think the theory that it's probably just parents telling their girls it's a man's sport most of the time appeals to me a tiny bit more. I don't however see any reason to straight out presume biology has no effect whatsoever, it's probably a complex mixture of factors.