This doesn't surprise me at all. Utah currently has the highest birthrate in the country, and according to the US Census, 61% of the state identifies as Mormon (I'm imagining that there are actually many more who are members "on paper only", who haven't bothered to resign).
Mormonism is a religion that not only encourages early marriage, but for members to have lots of children at a young age...and for the mother to stay at home.
A high percentage of stay-at-home moms in a region will create a greater wage disparity...even though they don't count a woman who isn't working to determine average wages, you have to consider that many of the women in the Utah workforce returned after a long time out of it. Less work experience + less education...