Grant (the engineer) asserts that all the mechanical factors will be the same for their full-scale tests of driving forward & reverse on ice. But the setup for forward puts the restraint chain parallel to the drive wheels' surface, meaning no change to the normal (downward) force on the front tires. The reverse test puts the chain angled significantly downward, meaning the tension on the chain adds normal force to the tires, which skews the force scale's reading of traction in reverse.