You have not been sold a genuine lid. Some parts companies sell a manually assembled cheaper hard plastic lid assembly, which has all original parts (screen, hinge cover, etc) except the lid, which is made of a grey plastic. Personally, I don't know why they make these, they look and feel horrible and I have never offered this to any of my customers as an option.
Send it back. For 400$, I would have thought they would fit a genuine one, since plastic ones are usually £50 cheaper over here.
FYI there are 3 grades of assemblies for Retinas available;
Plastic backing (like you have)
Minor defects (rejects from factory, but completely original. will either have minor marks on the aluminium or minor defects with the screen. the majority from eBay are this grade, even if they say 100% new)
Original (in a grey bag with black diagonal stripes. comes with new rubber hinge screw covers, and "gamma tested" stickers on the screen protector. exactly the same as Apple use, but difficult to get hold of)