The answer is correct it is
Die Deterioration. But why is this not from extreme die polishing? The affect of extreme die polishing removes the fields evenly:

Note on the image on the left, the affected areas are now flat looking. Note on the OP's coin the affected area is not flat looking but wavy. It is the die wearing away and flowing in the direction of the closest rim that is the difference. So it is die wear as the die is aging, not from abrasion to the die.