Perhaps it is just a simplification to meet the needs of a world-wide audience. Joanna Rowling is usually at the Edinburgh Book Festival so I'll ask her if I get a chance and feed-back her answer.

I heard a lovely story about why the play "The Madness of George III" was called the "Madness of King George" when it translated to film. It was because they thought that people would think they'd missed out on "The Madness of George I" and the "Madness of George II".