The quality of a part plays is a major in how much it costs t make something.
GM might be trying to hold 0.005" on the profile of a door, and 0.002" on hole locations.
Dynacorn may be holding 0.015" on profile and 0.005" on hole locations.
That 0.010" may mean that you need to spend an extra 10 minutes fitting up a fender: but it saves Dynacorn money in upfront tooling costs, die maintenance, engineering, etc.
GM wants to send a car off the line every 45 seconds, and keep the process as automated as possible. They may spend more money up front but they will make it up when they reach their monthly production goals.
