Obama's african ancestry came from Kenya and not from Georgia. He's afro-american but not black in the usual context used in the US. The exotic factor gives him a free pass that a black candidate wouldn't get.
So, racists only vote for black people as long as they have heritage from Africa and not America?
Obama is Light-skinned, lacking an accent, rich, not obviously 'ghetto/urban', etc. He's not the same as say Jesse Jackson Jr. or Harold ford.
Either of whom is light-skinned, lacking an accent, and rich. Jackson is also not really "obviously 'ghetto/urban'' - what he is, though, is linked (via his father) to the Civil Rights era.
The exotic factor didn't give Obama a free pass, but it certainly helped. A lot.