every user has a unique id, like the one in the picture, it's how the system identifies them all. when a account is renamed you don't need to change the settings in there as it uses the id instead of the name. however when a account is deleted it doesn't remove the item from the list, it just has no username for it anymore hence the strange item displayed. I'm guessing you deleted some of the builtin accounts (microsoft support) or reinstalled windows without formatting. It's not big deal just remove it as the account no longer exists.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS