I suspect just aerodynamics. When I pull my TT, the front wall is considerably farther behind the back wall of my truck and I think the air might be flowing straight back over the roof and beginnig to drop down the bed and then it hits the brick wall and has to flow in all 4 directions again to get around it. A 5w is sitting much closer to the rear window so the wind never makes it back down to the bed, it's just a constant stream starting from over the hood, over the cab roof, and then over the 5w.
There is a youtuber I follow, slim potatoehead, who just has a tiny jeep and little boxy fiberglass trailer. He always has interesting experiments and tests, and he mounted a snowboad to his roof rack as an air deflector and I believe he said he got significant improvements, like going from 12 mpg to 16 mpg in one leg of his trip etc.