I think if you wanted to get a better handle of how many women are in HN, a poll entitled "Are you female" with the options of yes-no-it's complicated- will give you better traction.
You mean: http://news.ycombinator.com/item?id=749617 ? There, female was about 6%.
The phenomenon simply is what it is -- I do not know or speculate on the cause. But I find it fascinating that many people feel compelled to quickly invent stories to explain it away. A little encounter with heresy seems to provoke a defensive rush into mythology.
I think a little cognitive dissonance, a little acceptance of heretical fact, a little uncomfortable admission of ignorance, is good for intellectual hygeine; I welcome and cherish odd little phenomena like this, even if--in fact, especially because--they run so counter to my beliefs and expectations.