That is because Obito presented himself to Kisame as Madara, when Kisame was still a ninja from the Mist, when Obito controlled the Fourth Mizukage Yagura. :y
I guess when Kisame first met Obito he told him he was Madara and made himself look like Madara. But what is weird it doesn't seem like Obito would have been that old at the time so its like a WTF moment.