I'd assume it was a spelling pronunciation, intended to represent
Arthur. And I'd feel sorry for the child and all the explaining he'd have to do.
An old lady I once knew said that one of her pupils was named
Yvonne and firmly believed that it was Why-Vonny. The assumption was that her mother had probably read a novel or a love story in a magazine, found the name exotic and appealing, and never imagined what it might really sound like. That would be my reasoning if I met an Author.
Where I live, some regional accents actually do make
Arthur sound like Author. I lived next door to an
Arthur once whose mother used to yell at him all day: Auth! Auth!