Good question.
That was the median employment incomes for elementary school teachers, so it's not taking into account differences in the number of hours worked.
What I would suspect is that the reason for the $10,000-a-year gap in salaries would have to do with that problem of rates of promotion, so, say, you see more men going into school administrator positions, which are higher paid. Also it's to do with the fact that in any sector, women tend to be overrepresented in part time, temporary, and contract work.
I suspect that in education you would also see an overrepresentation of women on contract or working as substitute teachers, essentially, so even though you have a highly unionized sector and some good policies in place to try to equalize pay, we're still facing challenges around promotion and around access to permanent, full-time work.