Obama's skill is with a script and a teleprompter. On his own, he's surprisingly weak and churlish. I think part of his problem with contradicting himself is that he doesn't remember everything he said in his speeches. He didn't write them, he performed them. He doesn't have to know what the words mean. It's enough to get the music and staging right, the JFK/MLK Jr. cadence, the uptilt of the jaw. Pure theatre.
He's so weak on extemporaneous stuff because his success beyond his core base of Marxists and Black Liberation fellow travelers has always depended upon illusion and masking what he really believes. That takes packaging and scripting, and he's just not able to do it on his own