When people delete their accounts, their name still shows up for each of their posts, but in black, with the descriptive "guest" under it as opposed to post count and stuff.
They're not coming back to post as a guest, that's not possible on this site. You either have an account and post, or you don't. The "guest" thing automatically happens when you delete you account.
And some people then "reregister" the name to prevent people posting under it. Some have taken deleted usernames, reregistered them, and proceeded to flame people.