Coins, tokens, medals (and silver strikes) with frosted surfaces are a result of the earliest pieces being struck from a new die.

Remember, the raised portions in relief on the finished struck coin, token, medal are recessed (sunken) on the die. The flat die surfaces are polished before being used which results in the mirror finish of the flat fields on the silver strikes. The frosted portions on silver strikes are a result of the recessed areas of the die that can not be polished such as the flat surfaces of the die are.

As the die wears due to metal (silver) being compressed into the voids of the dies, the recessed areas of the die becomes smoother and the frosting effect is no longer exhibited on the raised portions of the finished product of later strikes.

Hope this explanation helps.