I watched the show, Myth Busters when they addresses the idea of getting better gas mileage in a pick up with the tail gate off. For technical reasons I won't try to explain, it turned out that the truck with the tail gate got better gas mileage. It had something to do with the vortex of wind turbulence created in the bed of the truck. Soooo, only a wild ass guess on my part but maybe the higher back is part of the engineering that goes into todays vehicles to try and milk every small mileage increase out of them that they can?