I'm don't know that any real explanation is gonna be much more than speculation but it certainly is an interesting graph. I'm actually really surprised by the percentage of women in the physical sciences. I studied physics and sadly, I'm afraid that if you split out physics from the other branches it would be significantly lower than the other physical sciences. My own anecdotal experience (undergraduate and graduate school) would put it at 10%. But that is just two snapshots in time at two schools. I would guess the average HAS to be higher, but still below 40%.