It is often viewed that, as seii taishogun was appointed from the Minamoto clan by convention, Ieyasu TOKUGAWA borrowed a spot in the family tree so as to be a descendant of the Nitta clan, thereby calling himself 'Minamoto no Ason' so that he could become seii taishogun.
そのため、慣例で源氏がなるとされていた征夷大将軍に任官されるために、新田氏の末裔となるべく系図を借り受けて「源朝臣」を称するようになったという解釈がされることが多い。