I'll go ask Mr.Kishimoto,cause I have no idea.
Well,Kyubi once said Sasuke's chakra was just like that of Uchiha Madara.So,maybe without seeing a person's face it can't really tell who that person is.Tobi kept proclaiming he was Madara,ans maybe they have almost identical chakra(like Sasuke),so the Kyubi assumed it was Madara and referred to Tobi as such.