---note: this is not anything to do with actual capabilities!---
Regardless of what causes this difference (cultural, social, biological, monkey-based evo psych, whatever) I would bet it is a factor here. So I posit that men may cover up their lack of knowledge, but it may also be a confidence issue - perhaps the men actually feel like they know already, and the women are less sure of their own knowledge.
[0] https://en.wikipedia.org/wiki/Impostor_syndrome
[1] http://adainitiative.org/2013/08/is-impostor-syndrome-keepin...
I think you can genuinely believe that you know something until you are called out on it, only to realise that you can't even explain it.
If you don't know something just reply 'I don't know.' followed by 'Can you explain that to me?', you might be met with stunned silence. Or better still someone will have a go at explaining something. Sometimes the process of explaining it helps you cement your understanding. When working alone I really miss that type of interaction.
When I was a young boy, my mom always used to tell me that the true benchmark of whether you understand something is whether you can explain it to someone else. It's a great self-checking habit to form indeed, because relating knowledge forces one to organize it and whittle it down to essentials, and often new insights spring forth in the process. So I really frequently grab someone blessed with curiosity and just talk at them about what I'm doing.
Good old mom wisdom.
Where have all the smart hackers gone who're so quick to point out that society is at fault when negative characters of overly-stereotyped women are raised? If I were to play their role -- and tacitly accept this farcical charade that men are all bluster and ego -- I'd point out that the reason that some men might be less inclined than some women to admit they don't know the answer to something is that they know that they'll be held accountable for not knowing that thing. In this stereotyped version of reality over which we all seem to be forming a merry echo-chamber, women are unlikely to be brutally admonished for being wrong or incompetent (perhaps because those around them expect no better of them, due to negative stereotypes). This gives them the freedom to admit they're wrong, or that they don't know a given thing. Men, on the other hand, will be held accountable for their lack of knowledge, making it a much riskier endeavour to admit they don't know something.
Of course, this entire commentary presupposes that this phenomenon even exists. My team (half male, half female), shows this 'desire to be seen as knowledgeable' as a corollary of age and gender combined: the oldest member of the team never says 'could you explain that to me', and always attempts to BS his way through conversations, but the younger 20-something aged male members of the team are perfectly happy to admit it when they're out of their depth. Maybe this is unique to us? I doubt it: somehow I expect that this stereotype -- much like all other common stereotypes -- is overly broad and downright moronic.
I always know that when I have to talk to a group of women I have to be well prepared. Men are generally easy to bullshit, they won't ask questions like "could you explain exactly how that works?", at least not in front of their peers. Women seem to have less of an issue with it, especially when there are no men around to give them condescending looks.