Interesting issue, and I have a theory about this.

I think this may be because abdominal fat is deposited via a different mechanism than the other fat. Visceral fat is linked more to insulin resistance than other fat. My butt has shrunk disproportionaly too, the thigh/butt fat has a lot to do with estrogen receptors in that area, so I think that by balancing our hormones with this WOE we are able to lose that fat easier than on other diets, along with fat in other parts of the body. But the visceral fat is symptomatic of the very root of the problem, the IR, so it leaves slower and the rate of loss is maybe related to how IR we are, and how fast we can heal that on this diet. I know high cortisol can aggravate IR too.