I have a couple of theories. One is, he has a crazy ass hard name to pronounce and let alone remember, like a 16-syllable name. Another is his name is slightly uncommon but sounds like a regular name - e.g. Jat vs John. Or maybe she’s kinda flaky, like I am. If she’s done it too many times then it’s embarrassing to correct her. Boy, just accept the name and tell people it’s her pet name for ya ;)