What you are seeing is the effect of a huge vortice at the back of the cab. Vortices create big losses due to drag as was exampled by the sloped rear window. (Note: It's no accident the Porshe is shaped the way it is) Since the air is swirling very fast in a clockwise direction if you are looking at the drivers side of the truck, it is basically heading downward in the region of the tail gate. If the wind were directed right at the tail gate it would make a big difference if it were up or down but since is it more parallel with it it does not.

If you were to have a cover coming from the top edge of the cap to the edge of the tail gate while it is down, you'd likely see a large improvement in drag.

Since drag force increases with, amoung other things, the velocity squared, small changes in speed make large changes in drag force. This is why small changes in the drag coefficient make big changes in top speed.

Frank.