Ok, lemme get this straight in my head.
The point is at it's thickest somewhere just ahead of the base, right? So as the thickness of the point passes thru, for instance, the rib you pictured, it would open a passage that the sinew wrap may pass thru with less resistance? Ok, I can see that.
I think there was something in the Ashby research about how a shaft slightly smaller than the cross section of the broadhead suffered less from resistance/drag/friction from the wound channel and gained penetration.