I wonder if the participants were asked to confirm some of their "estimates." For example, "are you sure 90% of Americans are either Muslim, Jewish, or Athiest?" Or, "do you really believe 92% of Americans live in either New York City, the state of Texas, or the state of California?"
On second thought, considering how many of the "estimates" hover around 30%, I'm more than willing to guess a bunch of people just fucked around and put 100% for everything.
Edit: After looking at the full list, the second thought doesn't apply and I have now learned how to cross things out again.