because that is where the die that made your coin has started to wear. this may even be from the mint workers polishing the die. polishing a die will also wear the die away . I can see what looks like polish marks above the arrows. sometimes the dies are polished to remove unwanted marks like say clash die marks. you are doing good and I bet you learn fast.
Don't worry about this for now and as you learn more you then you will recognise what you have better. I can give you a hint that may help you . just imagine a hole that is filled in with polishing compound . what is going to happen to the top edge of the hole when a buffing wheel starts digging out the polishing or buffing compound ?
