Right on the nose! When I first got interested in trains - I had had an historical interest in railroads for a long time - the steam locomotive had only been dead for a couple of years and there were numerous editorial diatribes within the narrative of published articles bemoaning the
similiarity of diesel locomotives. Manufacturers do put out an off-the-shelf locomotive but few, if any, users buy it that way without customizing such things as bell location, horn location, headlight location, etc.
And even the road specific details can change between orders…Then the same railroad can have “early” and “late” phases with different details and perhaps different road numbers!!
To add on to what Brakie said - when an engine hits the shop for routine inspections or even minor repairs, parts from one engine can be snagged and put on others. Hand rails from a GP9 might be salvaged and modified to patch up a sideswiped section on a SD40 if they needed immediate turn around on the unit. No two engines after their first repair will be identical again.