Here is a look at the obverse where you can see that die was also polished harshly. Polishing is not really an error, nor is running the die in late stage to the point where is shows metal fatigue and flow, like the bulges and wrinkles around the reverse on this one. The error was probably clash which was polished off. As with any coin it is possible grease was involved, but it is not possible for the grease to add relief to the field on the coin as is obvious in the photo. That comes from removing the field on the die by polishing it or die fatigue/flow.
