The book is really good (though he sometimes doesn't know all the relevant facts but who would). His point:

There is a legal minimum wage. Of course, even if the legal minimum wage were not a factor, there would still be the de facto minimum of zero. Nobody works for a negative salary. A negative salary would mean, essentially, that if you were really incompetent you would pay the company to let you work for them.

Again assume that the average inputs are exactly the same for men and women: the same qualifications, the same abilities, the same efforts and sacrifices. Assume the average man and woman produce exactly the same quality and quantity of work. And assume that there are more men at both extremes.

There may be a minimum salary, but there is no maximum. And so at the high extreme, the top-achieving men can pull up the men’s average salary. Meanwhile, at the low end, the floor (the minimum wage and the zero point) prevents the low-achieving men from pulling the male average back down. As a result, men’s average salary will end up higher than women’s—even if the average man is precisely equal to the average woman on every single factor that determines salary.

Futhermore he argues that the same mechanism affects grades:

In many college classes, the majority of students get A grades. Failure is increasingly rare.

The highest possible grade in most colleges, or what statisticians call the ceiling, is an A. The lowest (the floor) is an F. Over the last couple of decades, all grades have drifted upward toward the ceiling. The floor is far from the average, and the ceiling is close to it.

The low ceiling will affect the average grades of men and women.

Remember, we have more men at both extremes of actual achievement.

The really high achieving men, however, can no longer pull up the male average, because of the low ceiling. There is no grade above an A. (Curiously, many universities do not even permit A plus grades.) Lots of people get A’s. And so the super-brilliant students get the same grade (A) as the moderately bright and conscientious ones.

Meanwhile, though, the low-achieving males will pull the male average down. There are more men than women getting the really low grades, and as these stand out more and more because of grade inflation, they will exert a strong downward pull on the men’s average.

The result? Women will get better grades than men, on average.