One might be boys' obsession with video games which eats up a significant amount of time in the 6-18 year old time frame, thus reducing time spent studying (or socialising, or playing sports, or dating, etc.).
A second might be that having been through a tremendous amount of hardship in their struggle towards equality and fairness, mothers might push daughters far harder than they push sons, as they know the deck is stacked against daughters. I think that this attitude, once internalised, stays with women through the rest of their lives.
Potentially a third factor is that with smaller families, mothers of sons may tend to spoil them more than they spoil their daughters - this seems to be the case in the families of sons in my circle of acquaintance, anyway.
These are simply observations, and may or may not have substance. It is interesting, however, that at my daughter's very good high school, something like 13 or 14 of the top 20 academic achievers were girls. Also, most of the top marks at her university were also from young women. We are also increasingly hearing that US universities are trying to keep the gender balance from going to far from even, and thus are accepting young men with lower scores than young women.
On balance or on average, it already seems that young women are smarter than young men, and I have known many exasperated young women who have to lower their level of conversation in their dating because the young men simply are not their intellectual equals. I find this very sad on so many levels, and it does not bode well for the society as a whole.
P.S. It is good to see you here as I have really missed the ladies of HoF. There used to be so many good discussions from which I learned a great deal. Hoping to see you more often.