I'm inclined to agree with you, John, about production tolerances. I recall picking up a Jetstream after a re-paint, checking the docs, querying the fact that the aileron trim was now supposedly neutral (previously this A/C had always been neutral with two notches left trim, to correct what I assume was a manufacturing tolerance problem) and being reassured that all the rigging checks were OK and the trimmer really should be now in the middle for neutral. Oh what a surprise we had on taking off for the check flight and finding that the aircraft had a fairly significant inbuilt roll, corrected by trimming back to the "normal" setting.........