Let me try to explain how it was at the dealer I was at.
The dealer ordered certain things each time on the cars and it did not matter if it was a 6 or an 8. He was looking at warranty costs on what it would cost to fix it.
Or that is what he customer special ordered when the car was originally bought.
Or that is the rear end they had at the time on the assembly line. Or the dealer knew someone that worked the assembly line and had it changed.
I have seen all of this happen.
I hope that helps
Gordon