Why I Laugh P3

So today a girl named Stephany helped me do some small task (pass out papers or something to that effect).  I said quite casually, "Thanks Steph."  A little boy a few seats down from her, Jose, asked, "Why did you just call her 'Steph'?"  I easily explained, "It's just a nickname for Stephany, it means the same thing as calling her by her full name if she's okay with it."  He said, with little recognition of just how humorous it actually was, "So what are you going to call me?  Jos? (pronounced as English word 'hose.')"