re: "Hah! I knew it." - Voldemort
Book authors and screenwriters often compile detailed biographies of even minor characters, to give depth and a "back story" to these players. Rarely does all of this background information actually make it in print or on to the screen, but it is useful to keep the character's persona consistent, and to aid actors in preparing for the role. I don't know if this is the case in Rowling's Dumbledork, but I would not be all that surprised if it was.