Most people put right thrust on their motor to offset the torque of the prop. When they do that, it pushes the centerline of the motor off of the centerline of the plane. If you put the motor on the other side of the plane and give it some right thrust, it tends to align the motor and plane.

Honestly, I mistakenly put it on that side late one night and never moved it. Sure is great for tuning the low end of the motor. Later someone did give me the explanation I first stated and the plane flies great.