Ideally, the reason people believes X instead of not-X is because, having heard the arguments on both sides, they find the arguments for X to be better than the arguments against X. So, when they judge a debate between X and not-X, as long as the debaters don't bring up things the judge has never heard before, it's inevitable that the judge is going to think his side made better arguments. After all, it's because of those arguments that the judge came to hold his opinion in the first place, so he obviously thinks they're more persuasive than the arguments from the opposition.
People in general usually make biased votes. They use their personal preferences over the facts when making decisions. This is especially true among DDO members. They seem to vote with their heart and not with their brain. I don't know if this epidemic can be fixed but I like to think it can.