Of course, Itachi and Obito have the best poker faces of the manga and they both had something to gain by lying to Sasuke.. Itachi would prevent Sasuke from trusting someone like Madara, and Obito would get Sasuke onto his side.
Or Obito actually thought it was the truth. Madara gave him the mission to take his identity, so surely he had to tell Obito his lifestory... and it wouldn't be surprising if he left out a few things, to keep the kid's trust.