Old news, I'm afraid. Boeing attributed it to the weight of the pre-installed cabin fittings and associated wiring slightly warping the airframe.
As the article goes on to say, once the composite fuselage sections are mated together, the internal structure will become 'load-bearing' therefore taking the excess strain off of the outer skin of the aircraft.
Titch
Vidi, Vici, Veni. I saw, I conquered, I came.