I think it's because the mistake version is at one remove, whereas in declaring someone to be "wrong" you are impugning their whole person. It's reminiscent of the difference alluded to by those who differentiate between 'hating the sin but still loving the sinner', or the example of the loving parent who marks disaproval of an unnaceptable action without loading the child with fear and self-loathing - "bad action, not bad person", if that makes sense?