Different incidence on the elevator halfs influence the knifedgetrim. The elevator half pointing up in knifeedge is partialy blanked by the fuselage. Therfore the lower elevatorhalf controls the tracking. This results in different trim in right ond left knifeedge position when the incidence differ.
In the old days, before computerradios allowed the luxury of electronic mixing, this was used to trim an aircraft that tracked differntely in left and right knifeedge. If the stab was`nt adjustable the elevatorhalfs neutrals were slightly separated. This proved very sensitive. On my Joker i wished it was possible to adjust the linkage 1/4 of a turn.

Regards/
Anders Johansson
Sweden