That microsoft ad is infuriating. Someone seeing that ad and thinking they have little chance in STEM fields because only 6.7% of women graduate in those fields ignores the fact that women get more STEM degrees then men in biological sciences. You know, like medicine, the field the little girl in the ad wants to study, to find a cure for cancer.
The biggest gap is in engineering and computer science. There is a gap in physics and other hard sciences too, but hardly anyone majors in those in the first place, of either gender.
The 6.7% figure, if not wrong, is meaningless, because psychologically it seems like a deterrent (those are my chances!) while ignoring the higher female college attendance rates (and graduation rates among existing students) in a wide variety of fields, some of them within the STEM rubric. STEM in fact is an arbitrary social construction, taking the scientific fields of the Liberal Arts colleges (chemistry, physics, math, biology) and linking them to the engineering school.