[AndrewJ]

yes, you ideally want a "clean break" at the rear, but the front edges should (again ideally) be rounded.

In this test I don't really think that having the leading edge being "boxier" would effect the outcome very much. Seems to me that the results are pretty clearly showing the effect of trailing wake.

With the shell on, the trailing wake is as big as it's ever gonna get, therefore the mileage suffers.

With the shell off and tailgate up, the trailing wake is smaller, and (I think) the air can best approximate the "ideal" flow, coming down from the cab at approximately 13 degrees, just clearing the top of the tailgate.
Clearly it's better to not have the air "approximate" it's ideal line, but to give it an actual form (aero cap) to flow over.

With the tailgate down I would think that the air is no longer being "fooled" into following the 13 degree angle, so it's just separating off the back of the cab much like it would separate off the back of the camper-shell.