The numbers reflect a few important things, the major one being that the isolationism created by the Watchtower leadership has had an amazing effect on their flock.
I think the one thing that is making the Witness numbers higher in this poll as opposed to all other religions is education. Every other group has allowed within it's ranks people to go to college, become scholars, publish articles, and pursue some sort of academic career. The Watchtower has not. It has rejected society as a whole and refuses to engage in the normal processes it uses to establish understanding about the world around us.
The Watchtower organization has essentially retreated from society, labeled itself all powerful and all knowing, yet refuses to engage society in any reasonable way to prove what they believe is true. This is isolationism. Their rhetoric can only be believed by people who believe that society is fully corrupt. Even the mormons have integrated with society. In fact, the poll shows that 20-30% of mormons are more in line with the views of society than the JWs.