Lie to the pollsters about voting for the black guy because you don't want them to think you are racist.
Remember that in SC, polls taken the day before the election had gay marriage winning big. Once voters had their opportunity to express their true feelings in secret, it was massive the other way.
No such thing happened in 2008. Why would it be different now? The so-called Wilder/Bradley Effect is historical (1980s) and applied to only subsections of the whole country (CA, NYC). Even then it is of dubious value.