Crummy aftermarket lid. Actually they are very good, for fit and finish.
For some reason all the Montes we have had, the trunk lid has a solid thud as compared to the Chevelle's we and friends have had.
That being said, MAYBE the Monte Carlo had a little more seam sealer like stuff added to the inside of the trunk lid, between the inner and outer skins. Before or during assembly. That's a big maybe.