Often positive and negative differences between the ‘right’ answer and the arrived at answer don’t cancel each other out.
There are many disciplines in which errors do not cancel each other out errors on the high side and errors on the low side do not cancel each other out. Instead the absolute value of the error adds up. As in justice done to a criminal who was given to light of a sentence and injustice done to a criminal who was given to have you ever sentence. You cannot say that each of them on average was given the right sentence. They were both given a wrong sentence.