AMT/SMP did some goofy stuff back then. Since the 1959 kit has been reissued, it would seem that the '60 was an entirely new tool which was then altered to produce '61, '62, and so on. I do believe the '61 has a separate roof for the hardtop.
It's amazing that Chrysler actually ordered Imperial convertible promos in those years, because they sold so few 1:1 convertibles. Then again, Ford ordered '60 Edsel promos when they knew they were pulling the plug on the car even before the '59 came out.