[Unreal]

Cars were not scheduled to be built, until all parts were scheduled for delivery. In other words, Fisher would not assign a body number and start a car with the expectation that the body would be pulled off to the side to wait on an engine, or rear, etc.
In the real world, though, sh!t happens. Delivery dates for parts or assemblies get missed. That's when mismatched dates occur. The rarer the assembly, the more likely the occurance. eg. BE rear.