I can tell you how it happens, but I can't tell you why it wasn't corrected before leaving the factory.
I learned, during a tour of the Corvette factory, that when the cars go down the assembly line they use "just in time" assembly. Meaning that the parts arrive at the factory as they are ready to be put on the particular cars. Sometimes they mis-judge quantities, or the parts are defective/damaged etc. In order to not stop the assembly line, they put any part on there so they can complete the car. Once it is off the assembly line, they put it aside for repairs before it leaves the factory. At the corvette factory I was seeing yellow corvettes coming off of the assembly line with red door handles, mis-matched removable roof panels, etc. Some even had minor body work in progress.
I have to assume that these things that were noted in the above posts are a similar scenario, but I can't figure out how it passed the final inspection, and not corrected before leaving the factory...no excuse for that!