The biographies may be unbalanced because they are being written by a second-hander( someone who was not there ). They did not actually experience the event, so they may not be able to retell the person's story with as much detail. That is why biographies may be biased, without very much details, or unbalanced.
A detailed secondhand edition of a person's life.
A firsthand edition of one's life; Written by the person themselves.