Hmmm.....from what I'm finding via Google, it appears the Q7 (among others) have hitches that are simply not rated for use with weight distribution hitches. I guess the load transmitted through the WD hardware is more than the frame attaching points can handle.
This is a pretty good reference for how WD hitches work. They attempt to transfer some of the trailer load into the vehicles suspension/chassis. When the tow vehicle is squatting too much in the rear, front steering and braking can become dangerous.
https://www.etrailer.com/faq-weightdistribution.aspx