If you are talking about the center of the mint mark being filled, that is from the post that forms the cent is breaking off the die. When it is totally gone it can even be taller than the mint mark. That is because the area the held the post in place has broken off lower than the post area. Same with the digits and letters that have a center. The post can break away from them as well:

When a
Grease Fill on a die happens that device can be partially affected, or totally affected, depending on how much grease had fill that device. (the grease prevent the strike from happening on the coin)
Edited by coop
09/18/2016 6:10 pm