Gender and Grades

Last week I was a teacher. Next week I will be a philosopher. This week I am a professional grader. So, a grade question.

Why are women better students? Not a universal claim, but on average my female students are better than my male students. A friend in admissions once said that if we went gender blind, the institution would be 75% female and this is the case almost across the board. Why is that?

Anecdotally, if I think back over my top of the top, A+, wow type students, there's gender parity. When I think about the next tier, though, the solid A/A- do really good thoughtful interesting consistent work, the women outnumber the men. What social factors account for this?