My off the cuff guess is that a few different things could be going on. One, boys may be more likely to be the squeaky wheel by misbehaving when needs aren't met. Two, boys may be more represented in math which tends to impress the verbal, nonmath oriented people more. There is a clearer path of advancement in math. Three, some parents may an unstated belief that school is more important for boys and getting along is more important for girls.

At any rate to me 60/40 is no big deal and I wouldn't give it another thought. In much of life people won't always be in perfect 50/50 environments. More women than men go to college and at many small liberal arts college especially that plays out to 60% women and 40% men.