Because it has a selection of protagonists is those who know all about this world and all those who know of it, he needs to lie about a fake pearls because they fear that would cause others trouble.
Because of Micah Lecter's character is the one who knows everything about the world and the need to know everything, so he lied about a fake pearls for fear of the expense of others.
Because of the Elector of the protagonists Micha is know everything on earth and being who knows everything so he had to lie about the Pearl as fake because they are afraid of making others in trouble.