Ima say after absorbing Orochimaru, the reson is because Sasuke and Naruto always have similar levels of strength and Kakashi stated Naruto surpassed him when creating the Rasenshuriken, which was before Sage mode. I'm not saying it is, but say sm Naruto = ms Sasuke pre sm Naruto would = roughly absorbed Oro Sasuke thus would have "surpassed" Kakashi.