Bob's in a dangerous profession. Viruses and other nasty people have serious distaste for Guardians. Therefore, it would make sense for Guardians to go only by their first names--or even an assumed name--to protect their families. Whatever Bob's full filename is, he'd probably keep it under wraps--and insist that Dot keep her maiden name if and when they ever get married.
In that sense, saying that Bob's last name is 452 is logical--most last names denote some kind of group tie--and it seems that only Guardians carry protocol numbers. So introducing himself as "Bob, Guardian 452" states exactly who and what he is--sort of like a police officer saying, "I'm Officer Smith, Cleveland Police Department."