The following documentary covered it and according to them it is not because bright girls are pushed to "girly" fields:

https://youtu.be/E577jhf25t4

Also watch the other episodes at:

http://www.youtube.com/playlist?list=PLd9_g7lAICxtlGbxh4_z8i...

https://youtu.be/Ve6uK00AvNo

It also covers why women in for example India and the Middle-East are more into computer science.

You can read more about the series at:

https://en.wikipedia.org/wiki/Hjernevask