In that case, I guess it's about the visible hair lines and the amount of allowed polygones inside meshes and maybe further works like with the texture.
At the moment, they have to let the hair float (with every race), to create an irregular hair line far easier. It's either that, or you let hair clip into the head, and need some editing on the head texture to create the natural and softer hair line - at the troubles of potential seams.
Quick graphical edit without further adjustments, just plumb clipping on right pic. Other two are original.
But why the gap is that big around human-like head shapes, which aren't that different between selections, compared to Hroth head? No idea. At least there, they could almost fully prevent those, as the skulls have no deforming animations above the face area.
And with that topic, Square did a better job around the racial hair styles on Hroth than the other races. I have to take sometimes very odd camera positions to see a visible gap, while with others often even a plain view from side is far more than enough - even with their unified shape above the forehead.