It's part of his leveling, "aw shucks" style, which lets him say he's smart, and his audience is smart for going along with his humor. I think it's rather effective (alas).
From AsiaTimesOnline
Feb 26, 2008
(snip)
Barack Obama is a clever fellow who imbibed hatred of America with his mother's milk, but worked his way up the elite ladder of education and career. He shares the resentment of Muslims against the encroachment of American culture, although not their religion. He has the empathetic skill set of an anthropologist who lives with his subjects, learns their language, and elicits their hopes and fears while remaining at emotional distance. That is, he is the political equivalent of a sociopath. The difference is that he is practicing not on a primitive tribe but on the population of the United States.
There is nothing mysterious about Obama's methods. "A demagogue tries to sound as stupid as his audience so that they will think they are as clever as he is," wrote Karl Krauss. Americans are the world's biggest suckers, and laugh at this weakness in their popular culture. Listening to Obama speak, Sinclair Lewis' cynical tent-revivalist Elmer Gantry comes to mind, or, even better, Tyrone Power's portrayal of a carnival mentalist in the 1947 film noire Nightmare Alley. The latter is available for instant viewing at Netflix, and highly recommended as an antidote to having felt uplifted by an Obama speech. ..."
Article: Obama's women reveal his secret
http://www.atimes.com/atimes/Front_Page/JB26Aa01.html