I've been looking around for a developer recently, and in looking at the profiles of women vs. men at places like AngelList I noticed a few key differences in how they described themselves. We're talking about people with very similar academic backgrounds here with similar levels of experience.
Women would say things like "I worked on a team that did X," while men -- who obviously did the same kind of work -- would simply say "I did X." Women would highlight the cooperative nature of their work, while men would give the impression that they did it all themselves. In my experience the former is typically more honest, especially if you actually were working on a team or on a team-oriented college project.
Women also tend to highlight team-oriented and non-technical skills more than men in general, which to techie-minded hacker types gives the impression that they are not strong on the technical end.
I also noted my reaction to these differences. Introspecting, I noticed that I tended to assume that "being on a team" meant riding the coattails of the other team members. If I noted this bias in myself, it's probably pretty common.
I decided to try to de-program myself of this bias by thinking of all the counter-examples I've seen.
I do believe in the "10X programmer" -- for the simple reason that there are 10X everything elses. There are 10X athletes, 10X businesspeople/salespeople, 10X artists, 10X anythings. When we look for employees or contractors we are looking for that 10X standout, or at least someone who's going to grow into one. But being really good at something is not mutually exclusive with working on a team or highlighting non-technical skills, and the inverse is also not correlated. I've met plenty of monomaniacal "closet programmers" who really aren't any good. They sit alone stewing in their huge egos and churn out crap. (But then again I've met some who are. There does not seem to be a strong correlation.)
There's probably quite a few really talented female programmers out there who are being passed over because they don't conform to the "whip it out and let me measure it" customs of technical recruitment. I also suspect that this does drive women from the field.