I work in Supply Chain. All you're probably seeing there is the difference between suppliers.
When a vehicle is in production and plan requires, say, 5,000 pc/week of some trim part it is not unsual to have several suppliers. That's a decent volume and a number of suppliers would be interested in a piece of the business. Sometimes you need more than one source as seasonal demand peaks may be more than one can handle. Some companies want a source within 1 day shipping from an assembly plant. So the source for rally wheel caps for Van Nuys may have been different than the source for Norwood. Cost is a very big deal to automakers. Even during production stuff is shopped around for a better price. Suppliers can be dropped for quality or delivery issues.
Service parts are a separate consideration. Service parts have to be individually packaged and high volume suppliers many not be set up to do that. So GMPD may designate one of the suppliers to send a number of caps per week to a packager. The cap supplied may not be an exact match for the original.
When a part is no longer current MY all bets are off. Volume drops off dramatically and the suppliers that were doing 5,000/week are often not interested. They had a line set up to do your part that is no longer justifiable. So now they have to set up and run a small quantity maybe once per month. Instead of a set up charge amortized over 250,000 parts the same costs will be incurred for 500. And it has to be packaged. Not unusual to see costs triple or more when they go non-current.
