I don't think this is due to learning.  I think it's an extremely conscious decision to always appear 100% confident once your certainty reaches 51%.

Suppose there's an issue, and you think you know what the right answer is, but you just can't be sure. And let's face it, Morris had no idea in March, and he knew he didn't.

As a political (and television) matter, it plays better to simply change your mind more often, speaking every time with complete confidence as if you've always said the same thing.  To most onlookers, this (probably for psychological reasons I'm incompetent to probe) appears better than to carefully consider things, admit that various solutions each have pluses and minuses, and that sometimes you can't know the answer but you have to try what you think is best.

As a policy matter (i.e. in matters of dealing with reality as it truthfully is), your commitment to a policy should generally track your ability to be certain it is correct.

I think both pundits and Republicans have mastered the political tactic.  You can be sure that if the Iraqi government asks Bush to withdraw, he'll act as if he had always been for withdrawal.  And he'll get less flack for "being weak" than if he had said all along he was staying but might change his mind based on conditions.