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I'd also question the PM results.
Assuming tailgate down initially without any topper or cover, there is turbulence, really a low pressure area, right behind the cab. Adding a cover splits the low pressure area into two, one behind the cab and another behind the tailgate. Adding a topper moves the lower pressure area completely to the back of the vehicle.
As PM mentions, this lower pressure area is effectively a parachute being dragged behind the vehicle. With a topper, it's a larger area and thus a larger parachute. With an aerocap, the low pressure area behind the cab is much less, as the air behind the window is all directed without turbulence as Bondo's tests demonstrated.
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