Given the fact that Itachi asked Obito to help him eliminate the Uchiha, why was he surprised when he knew about the secret mission?
Itachi may have asked Madara's help but it was never explicitly said that he revealed the true reason behind his actions.


  Itachi may have lied to Madara like he did to Sasuke that he massacred
  all uchiha to test his powers.


Itachi may be thinking that Madara was convinced with his reasoning, while Madara knew the true reason that he was doing all this by leaf village's orders.