I wonder how they accounted for volumetric differences-- brains have structural differences between genders, with male brains usually 8-13% larger overall, but it's a lot more complicated than that, with volumes and neural densities varying across many different regions: http://www.sciencedirect.com/science/article/pii/S0149763413...

This would trivially seem to affect blood flow, since blood vessels for different sized regions would behave differently.