Hi
Could it be down to quality? Quality departments typically work on parts per million. That seems a huge number compared to a run of say 502/504 as has been quoted, I've also seen runs of 512 else ware (maybe not model railways, it could have been die cast cars)... but what the manufacturer never says is how many is initially actually sold and how many they may keep for if something goes wrong.
My guess from reading this forum everything that comes in from model railway manufacturers in this country will have at the very least one item sent back with a problem.
The manufacturer then has a little fall back - not much but some.
The items that are then sent back to them can have other parts reused as we know has happened before. The company would probably write the cost off rather than the long and costly experience of getting a credit note for something that may have cost £30 or £40 to make. If you take into account it could be a motor that was bought in from a subcontractor it gets more complicated.
ie if Oxford get 504 locos and 2 get returned to them with say dodgy motors they could probably ask for a credit note for faulty parts from their unit in China, who then in turn ask for a credit note from their supplier that may even be somewhere like Taiwan.
Regards
Stuart