So, the way that vehicles are assembled is that they travel along an assembly line, right? And parts (like headlights and seats) are bolted on as they go? Right.
The hitch is that the parts that get bolted to each vehicle are on automated delivery systems of their own... it's not like the "seat station" has a pile of seats to choose from, and the guy then has to pick four that match. The seats are delivered on a track, and the dude's job is to grab the seat that's on the rack in front of him, and bolt it into the truck that's in front of him at that time. If it matches, great! If not, there's not a damn thing the "seat guy" can do about it... he doesn't have the option to do like a dry cleaner and scroll through what's available. So, obviously, in this case, the ****** occurred further up the line (from the assembly process), where the parts for that particular build were staged, and loaded onto their respective delivery systems (or the matching headlight assembly was dropped/broken and the next available unit was bolted in).
What's supposed to happen, is that there is supposed to be a post-build/initial quality inspection to catch and address these sorts of issues, but evidently a failure occurred there, as well. Which sucks... but at least the dealer is taking care of you.