1) there is a point at which a distribution skews so much one way, that it at least warrants discussion.
2) By "different fields", I assume you refer to technically driven fields. Though I do agree that some fields probably do sway towards one gender more than the other, there is a wealth of research that indicates that the astonishing lack of females in almost all technical related fields is probably due to a number of factors that can be accounted for and adjusted to. [1]
[1] http://en.wikipedia.org/wiki/Women_in_computing#Factors_cont...