Mark Blumenthal explained it all in a looooong post a few months back. Basically Gallup undercounts minorities. Pew Research on average estimates the black/hispanic vote to be 24.8% of the electorate, while Gallup estimates 21.4%. If you recalculate the Gallup Obama approval rating based on Pew Research demographics then the house effect is effectively wiped out.
http://www.huffingtonpost.com/2012/06/17/gallup-poll-race-barack-obama_n_1589937.html?utm_hp_ref=@pollsterOr I suppose you could argue Pew Research overcounts minorities, whatever you choose to believe.
