From what I was told in a chat room Counter-milling is what is done to the chamber of the pins to give false sets. If you look at the 3 left holes, you can see a ring on them which is the security feature that gives the false set. Here is a picture to show it.

Now people who search for this in the future(just as I did), will have the answer in the post

If this is wrong, feel free to correct me, but I was unable to find this information from searching and had to ask someone who told me.