I would imagine this is a pretty misleading poll. It has a lot to do with how you ask the question.
If you ask "Are you sure that Obama was born in the US?" You will get a lot of, "he probably was, but I am not sure".
If you are looking for a certain result (Republicans are racist and think Obama is Kenyan), you mark that down as a negative.
If you instead ask "Do you think Obama was born in the US?" and the answer is "He probably was, but I don't know for sure". Your answer should be marked as a positive.
It's not so hard to make a poll support what you want. Most Americans barely know who their senators are and do not have really well formed opinions on a lot of things. All you have to do is manipulate the doubt and you can make the poll do good work.