To expand on this: Confidence intervals that are accurate for multiple judgements by the same person may be accurate for the same judgement made by multiple people. Normally, we can group everyone's responses and measure how many people were actually right when they said they were 70% sure. This should average out to 70% is because the error is caused by independent variations in each person's estimate. If there's a systematic error, then even if we all accounted for the systematic error in our confidence levels, we would all still fail at the same time if there was an error.