Just about Ford Parts
Consider, average Ford tooling, varied by complexity, was 50,000 to 65,000 pieces. There was not a single tool stamping all the fenders, etc. at one time, and many parts were made of multiple pieces welded together, so one piece could be off a new tool, and another piece off a tool reaching end of life.
Then there was the issue of who was welding parts together, was it 1st day of deer season, 3rd shift with the guy 1st day on the job, etc.
Line rejects: I have owned NOS parts that were obvious line rejects. Why throw it away, when it can be sold through the parts department.
Passage of time: Some parts, especially chromed, turned to garbage as the length of time listed as a replacement part increased. Bumpers and back up / parking lights looked like they were chromed with gravel in them, no prep, no cleaning, just slap on some mediocre chrome on a poorly stamped out part. For example, I caught a stash of about 10 - 12 65 66 bumpers, all in NOS paper, in an old stash. Only 2 were worth putting on a car without work.
Rewraps and fakes: The rust red primer with yellow part number markings are a favorite. Take a repo fender, run it through a quick strip tank to take off the black paint, respray it rusty red with the yellow part number, etc.. Put it out back in the summer sun for a few months, and wrap it in some old Ford paper. $120 fender turns into an $800 fender, off to the swap meet or Craigslist.
The other issue is the unibody itself. 55 year old car, 7 wrecks, 230,000 miles, some rust, on a 5 year life expectancy mass produced car, and NOTHING is going to fit well.