From what I can tell, there arn't enough princesses to have all the characters at the meal there. Out of the six fairy tale Princesses, Ariel can't appear at either because of her fin, and Cinderella rarely makes it to Storybook Princess breakfast because she and Prince Charming have too many duties at MK to get to EPCOT often. Since Mary Poppins is a Disney heroine who wears a fancy dress, she often appears at the meals. Mulan has also been appearing at the Storybook Princess Meal as well and she's not a Princess either.