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