Spiral airflow would seem to be the culprit. On many airplanes, the difference in pitch trim in knife edge can be eliminated by changing the incidence of the right stab relative to the left stab. Decrease the incidence of the right stab (lower the leading edge) or increase the incidence of the left stab (if you do both, the elevator trim for level flight will not change). This type of change is more effective on some planes and setups, and I have never seen it cause any problems so far as loop tracking. This type of change is generally less effective when the plane is tailheavy.

Regards,

Dave