1)Someone could have removed the steering wheel at some time for other maintainance (dashboard, tumbler, etc.) and mis-installed it. (Simple solution).
2) It was aligned w/o using the centering bolt in the steering box and therefore both tie rods are off by an equal amount so that it tracks straight but is not centered. (Not as simple).
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)