Just a query that I came across when was searching some answers. The OP in the following Link is greyed out. Any particular reasons. I cannot even click on that and visit his profile.
This user was deleted before they anonymized deleted user names. At that time, all user deletions were done by a developer (more than likely because they were fairly uncommon). Now with the spammers and a vastly larger user base, things have become a little more standardized.
@Wont describes it more in detail, but it basically breaks to "There are two options: Delete and Destroy." These two options include either having the user name anonymized and the content remains, or having everything completely removed.