You probably know that the door is assembled to the car for [painting etc. It is then removed and goes on its merry way to separate parts of the factory to be fitted out. The main car body is easier to fit out without the doors in the way.
So later down the track, the supposedly correct doors magically appear after the fit out, ready to be put back on the body. Most likely scenario was 2 doors of the same (paint) colour on the assembly line were mixed up. Cloth went on yours and quite possibly, another cloth car has your leather door.
Very surprising no one noticed, probably because no one looks for such a thing. Too busy looking at paint finish, blemishes, gaps & the like. I doubt door swaps happens terribly much, if at all - except in your case.
328i M-Sport, Estoril Blue, Carbon XP 35 Tint, front heated seats, black Dakota, Sports Suspension, Anthracite Headliner, HUD, Pro Nav, Sunroof, hk 600w, Bi-Xenon, 19" 403M wheels, Reversing Camera, DAB+, Xenon, BT Ext Connect. JB4 on Map 2 with Catless DP produces awesome performance in Sport.