I'll check with people who worked on the production line. My entire family is retired GM blue and white collar.
I don't believe such an oversight can be made and pass quality control on a regular basis. My uncle worked quality control at the GMC Truck Center during the late 1980's and earlyl 90's and filled me in on this phenomenon.
GM was very specific as to where that trunk emblem was placed, down to the 1/16". GN emblems were not reported to be in short supply during production so it's doubtful they ran out during assembly, then reattached later. Another possibility is damage to the vehicle requiring factory repair prior to dealer delivery. This too would somewhat rare to have the deck lid repaired on so many brand new cars.
Unless an owner can say they bought the car brand new from a dealer I'd suspect repaint or specific emblem removal/replacement for some reason.
Repaint is fairly easily detected on GN's unless it's done by Earl Schieb. It's hard to reproduce the factory orange-peel and hairline scratches. Factory Grand National paint SUCKED. The section of my deck lid that has the emblem goes from horrible to terrible orange peel....not reproduced for an attractive re-paint.
I know cars weren't manufactured perfectly, errors occured and the GN was no exception. My 1st GN's dash plate was for a standard Regal. I owned my GN many years before someone pointed out the fact it didn't say "Grand National" but instead read "Buick Regal". I wasn't a GN aficionado at the time and didn't know it was incorrect. I've been told by several sources that one of two things may have occurred. They may have run out of specific plates that said Grand National or it was a drunken error and quality control missed it. That's much different than an emblem on the wrong side of the deck lid.
My $.02