It is rare to find that all the aircraft in a fleet are exactly the same because they tend to get built in quite small batches and there are inevitable changes in personnel and working practices. The dimensional variation is normally quite small but will give rise to individual idiosyncracies in trim etc.
When the Tristars were converted to tankers the UK contractor built an elaborate rig to install the underfuselage hose-drum unit. It failed to work because none of the aircraft were true to drawing, and moreover were all different to one another. At one stage the 80-year old head of the company was laying on his back on the hangar floor trying to kick the thing into position. Apparently Lockheed had been through a phase of hiring labour of dubious immigration status at the time and quality control had suffered.