My understanding of the summary is thus:
The variance in aptitude in men is greater than the variance in women, even though the average point for women is higher than that of men.
To put this another way (vastly simplified), there are more male geniuses and more male idiots. However if you were to grab a random woman off the street, she is likely to have greater aptitude than a random man you grab off the street.
So in US highschools where grading is bounded and the same testing is forced on everyone, we see women routinely trouncing their male counter parts in GPA scores.
Then in college, where people have a choice more men go into difficult but unprofitable majors (e.g. Physics), additionally women have a bias against certain subjects (e.g. Computer Science) due to the low-status accorded to it, and thus choose other majors like Law or Medicine.
----
Bringing it back to Genetics, that means the 'dum dum gene' (gene which causes aptitude variance) is on the Y chromosome, thus women aren't affected by it at all.