On average, this suggests that the true YECs are only about 10% of the American population (31 million people),
I find that hard to believe. That is, I bet most of the people who qualified as a YEC because of the poll they took, would not say they believed it if they were pressed with the ramifications of the belief.
Its easy to check a box in a poll. But to tell someone to their face that you believe something as incredibly stupid and unrealistic as YEC, especially in light of all the ridiculous beliefs that you'd have to bring along with it, is something that a very very small number of people would be willing to do.
Most people who self-identfy as YECs don't have a clue what all the implications are. Even the most prolific creationists, like Faith, have to conveniently ignore most of the implications.
That's part of why I cannot accept that they actually believe it. Sure, you can easily check a box in a poll and say you believe something, but that's a lot different that actually committing to it and accepting it as an thought-out truth (and bringing along with it the necessary implications).
It doesn't take much introspection to realize that an actual belief in YEC is ridiculous and unacceptable. But that's more than people are doing when they take a poll and go: "Hey, I like my beliefs on God, I'll check this box".
That's why I can't accept the results of a poll like this as actually representing anything that people have thought through and come to a belief in. It tells us more about the kind of knee-jerk reactions people will have when proposed an alternative to something they are uncomfortable with (or that they'll just choose the God option regardless of the ramifications).
People will say anything, but that doesn't mean they actually believe it.