Because the presupposition is that testosterone exposure in-utero is to blame. Without actually addressing this claim by medical professionals, everything else

Because the presupposition is that testosterone exposure in-utero is to blame. Without actually addressing this claim by medical professionals, everything else is manipulation of data to prove a point to address 'social policy' or an economic agenda. Moreover, the sample size, location, and dates of birth, are small and narrow, comparatively speaking. You cannot make sweeping claims like this using such small data set. But they do, and the "Material and Methods" section of this paper goes to great length to explain how they've made these extrapolations. This is statistical sausage. I have no doubt you could write a similar paper that claims the opposite; that females with male twins do much better. Just like this team did, you can find, extrapolate and present data to support the idea.