Kate Middleton is listed as a princess on Prince Louis's birth certificate — but is that correct?

On Prince Louis Arthur Charles of Cambridge’s birth certificate, Kate Middleton's full name is written as Catherine Elizabeth Her Royal Highness The Duchess of Cambridge. And then, her occupation is listed as Princess of the United Kingdom. Yet because only Queen Elizabeth’s children or grandchildren can be given such a royal title, it’s confusing why the Duchess of Cambridge (who, unlike Princess Diana, did not take on the title of princess) has been given one.