Take a look at '92 RT10's and compare prices. Same reason. It's because it's the first year that those were offered.
Similiar to the 97(?) RT's. It was the first year that they had the GTS options and not many were made, therefore a premium.
However, it's only worth that to people who care to pay it. I want to DRIVE my car, so I'm going to get the best deal I can instead of worrying how much it will be worth next year...