To explain very quickly and generally - a lot of men in CS (and probably in general) tend towards overconfidence, where women with the same skills will be underconfident in themselves. When you're surrounded by people who are ultra confident and professing their love for the subject matter, it's easy to feel that you don't love the topic as much as the people around you, and that you must not be as into it as everyone else. Probably best to read the referenced materials if you need to see these things quantified, and there's likely also other aspects that I'm missing, but that's the gist of it.