This is essentially the dirty secret of the tech industry, and its impossible to talk about because if you bring it up you are said to be sexist and no further discussion is allowed anymore.
I would submit its just part of the pipeline problem - women keep getting funneled out of tech at every stage from 1 year old to 30 years old, and so when your baseline group is only 20% the size of the other group (women and men) then of course finding the 10% of the group that is the absolute best in the field is going to stack the deck heavily towards men. Its 1000 men coders, 200 women coders, find the best 10, chances are they will all be men. Need it to be 500/500 to have a good shot at equality at startups and Google, Amazon, Facebook, etc.
The pipeline discourages girls at every step. Boy is into computers and video gaming at 5 years old, thats cool and his friends do it too. Girl is into computers? Eww, you don't want to grow up to be a nerd do you, nerds don't have boyfriends and don't go to prom. At 10-16 girls are discouraged in maths and some sciences (but not biology, weirdly) by their parents, peers, teachers, television, magazines, movies, everything. Girls are taught by everything they should care about taking care of others, animals, the environment, changing the world, etc. Boys just wanna play and code video games, race cars, and play sports and that is all heavily encouraged (to be fair boys are discouraged to be nurses, dancers, and other "feminine" careers).
Finally first day of computer science college classes, and a quarter of the boys are gonna be programming whiz's who have coded android apps and know perl, java, C++, can convert binary to hex easily and are incredibly intimidating to everyone around them. Combined with the coursework which is HEAVILY mathematically oriented the first year and it will cause a lot of people to drop out, including a lot of women. Sure, sexism and bigotry play a part every step of the way but this is not a problem human resources at Google is going to fix, its a problem parents and teachers have to fix among themselves but also peer pressure among girls has to change to not discourage "geeky" things. My daughters stopped playing their Nintendo DS's around 11 years old due to peer pressure and every kid's desire to be seen as cool by their friends.
Of course the sexism within the industry needs to be stamped out so girls who do graduate feel like its a place they belong and can thrive in, but just doing that alone probably won't change much.