It is not uncommon for planes with a few hours on them, and a few heavy checks and hard landings etc. behind them to be slightly out of rig. I was once non-revving on an old 737 and noticed, moving back and forth across the aisle, that both ailerons were up slightly, and one just a fraction of an inch higher than the other. There was no spoiler float going on at the moment.
After the flight I asked the captain about it. He was surprised to learn about the aileron neutral setting but said that that particular airframe had to be trimmed up manually at cruise. He had to punch off the autopilot, then trim the rudder, then the ailerons, then could put the a/p back on and it would fly with the ball in the middle. Without doing this it would not.
This was one of the early 737s that had the spirit level mounted above the ADI in addition to the "ball" beneath it. They had been using both of these instruments in trimming.
I would assume that in a D-check or the like, that the flight control rig settings would be put back to factory specs. Maybe I am wrong about this. But anyway, if they were, and if the plane had a slight wing or tail bend as a result of turbulence or something, then the factory settings might not make it fly straight and true.
I believe that this may, in part, account for the difference in cruise efficiency seen between older examples of the same type.
Happiness is not seeing another trite Ste. Maarten photo all week long.