I think the reason is because the 59 Mantle is in the 1st series (Card #10) and the first series of 59's do sell for a slight premium or at least they did when I collected the set. It is rare that the first Topps series sells for a premium but 59 is/was an exception.
My theory is that sales of 58 Topps were in the tank because the quality of the paper was slightly better then toilet paper and Topps cut back at the start of the 59 season. But that is just a theory
