Maybe the decals on the car are the "standard" decals that come with the Z-28 option and the other stripes that we normally associate with a Z of that era were actually optional?

I don't know for sure but I do know that the big hood bird that is usually on that vintage Trans Am was a quite expensive option but almost all of them got it and if it wasn't already on the car it was added by the dealer to "up-sell" it when it got on the lot.

Like I said, I don't know for sure, this is only an educated guess based on the Trans Ams.