Here's the thing with many N scale cars with body mounts - the coupler sits on the underside of the body, which may or may not affect the height of the car. On the prototype, the coupler doesn't actually sit on the underside of the body (though it might look that way) -- it sits at the ends of the structural spine of the car. Take a look at where the coupler pocket sits at the end of this flatcar: There's not much body thickness between the deck and the coupler pocket, because the coupler sits within the structural spine of the car. Most N scale cars are manufactured with a body, a weight that goes under it, and the underbody, by which the coupler pocket sits on the underside of the underbody. This is the reason we have unprototypical underslung couplers to make up for that height differential. So a lot of this has to do with the inherent design of how N scale rolling stock is designed.