I think you also have to take into account how many people are going to claim they believe every word of the Bible to a pollster because that is what they are taught to believe. Very religious people or people who grew up in very religious households may have a very difficult time admitting they have doubts about certain aspects of the Bible to a stranger on the phone.
One of my coworkers grew up extremely religious and did not admit that he was not completely sold on it until he was in his 20s. He is still religious, and still goes to church, but thinks his family is a bit too extreme.
So he would have given two different poll results if you asked him a few years apart, but he actually never really changed his beliefs. He just stopped thinking he had to say what they say.
