Obama's quoted statement referred to a possible decision to launch a unilateral U.S. strike on Iran, and the considerations in such a case are different from those involved in a multilateral, UN-authorized intervention. Still, it's possible that Obama meant that a president needs congressional approval for any use of U.S. forces that does not respond to "an actual or imminent threat" (or, like the 'Afghan surge,' relate directly to an ongoing conflict). If so, he would not be the first president to have said one thing about the scope of presidential power during a campaign and then to have discovered, once in the Oval Office, that he found a somewhat more expansive notion of executive power to be congenial.
As compelling as the gender split is, it’s even more interesting to look at the parallels between Obama and W. Candidate Obama said about a possible strike on Iran, “The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.”
Presidential systems, as opposed to parliamentary ones, seem designed to encourage a certain amount of gridlock stemming from the almost constant tug-of-war between president and legislature, especially in periods of divided or semi-divided party control (as is the case now, with the Republicans in control of the House). And congressional action can frustrate what should be properly be executive branch decisions, as in the case of where to put Khalid Sheikh Mohammed and his co-conspirators on trial.
But to come back to the quote on Iran, I'm inclined to think that Obama said it in the context of that particular issue and probably meant it to be less sweeping than the actual words themselves would suggest.