All dynos can be manipulated pretty easily. With Dynojets, people change the inertia values for the rollers to inflate them. I've seen different dynojets read differently as well.
Manufacturers rate their power based on engine dynos not chassis dynos. 18% loss seems low to me for AWD since most RWD's are in the 12-15% range. But again, it's all just numbers. I think the only good way to check a dyno calibration is with a precisely rated electric motor with no gear box.