Semi, Just looking at the pictures of the 3 guns you posted in this thread. The last 2 have butts that are nearly at right angles to the barrel & the top of the stock appear a bit more parallel to the barrel. The butt on the underhammer is angled forward at the bottom, plus it is very straight. My thought is that the flat butt and that angle would act as a wedge when you shoot & drive that slightly steeper top of the stock right up against your cheek. A bit of curve, similar to the other 2 guns would probably help anchor the stock to your shoulder.
I don't claim to be an expert in stock building, but that just seemed to jump out at me when I looked at the pictures.
Paul