I agree even a small amount of RPM change can have an effect... RPM difference may cause a slight change in flow, i.e. downwash angle between wing & tail.
Saw this on an A-6E flight test... back in the day
...
My other thought was that the airplane might just be built...crooked? are the other control positions instrumented? how do the rudder / aileron positions compare between aircraft?
I'd also agree to try to duplicate with same tail trim settings...
Frank