I refer to Jack as Jack in my e-mail. His e-mail is email@example.com. His resume lists him as Jack Lastname. So why does The Incompetent Recruiter insist on calling him John in his response? Frequently, because TIR is one of people who constantly works your name into e-mails.
I don't know if the fact that my usual problem is people coming up with wacky nicknames for me makes this more or less annoying.