There's really only two. He has a different father or he wasn't born in Hawaii. The first one would confirm his U.S. citizenship since just about everyone believes Davis is his father and the second one precludes his being president.
He also could have an entirely different MOM -- perhaps his birth mom gave him to his babydaddy Stanley Dunham whom he frequently looks like a replica of.
Then Stanley Ann helped out her parents and took him away to raise him.
Isn't it ... possible that individuals who were card carrying members of the CPUSA could have also been "swingers" and if Ann's DAD is the real father, maybe nobody truly knows who his baby mama is......