Why only 70% believe?

The South Koreans reportedly conducted a survey on the sinking of Cheonan and was surprised that only 70% of the South Koreans believed that it was the work of the North Koreans. Why is it that there are still 30% of the South Koreans disbelieving the claims by their govt even with their one sided evidence? Given the overwhelming proof produced by the South Koreans, the American and British experts, there should not be any doubt to believe otherwise. So far the western world all believes that the North Koreans are guilty beyond any reasonable doubt. Even the UN Secretary General said so, and he believed everything that the South Korean govt said. And reading from the media, it is conclusive that it was the North Koreans who did it. Would a one sided evidence produced in secrecy by a gang of interested parties that have all the reasons to point the fingers at the North Koreans be admissible in court as valid evidence? And China is expected to take these evidences as the gospel truth. Though China and Russia have said that they would want an independent investigation on the evidence, how much can they differ if the evidence were expertly cooked for several months? The happiest person is the little girl who thought that everything has been sewn nicely and foolproof, that the North is guilty and no one can deny it. The possibility of it being a North Korean act cannot be ruled out, but the probability is low. What if it was a sinister plot by a third party to fan the tension and at the expense of South Korean lives, and the South Koreans swallowed it hook, line and sinker? Isn't that a shameful tragedy while the third party or parties quietly partying away for a great clandestine op?