During the reign of Song Taizu, hundreds of surnames were written, and the process of Song Taizu's reunification went smoothly. However, the Northern Han regime, entrenched in the land of Sanjin, with the support of the Khitan, always caused a lot of trouble to Song Jun. It was not destroyed by the Northern Han Dynasty in the Three Signs. In 979 AD, Song Taizong destroyed this army. So the Song Dynasty hated the Northern Han Dynasty, and it happened that the ruler of the Northern Han Dynasty was surnamed Liu.
Moreover, the establishment of the Northern Song Dynasty was due to the abdication of the later Zhou Dynasty, and the regime of the latter Zhou Dynasty alternated with that of the later Han Dynasty. Only by opposing yourself with the later Han Dynasty can you show that you are the successor of the later Zhou Dynasty and the political power is legal. Moreover, the ruler of the later Han Dynasty was surnamed Liu, so the Northern Song Dynasty suppressed Liu.
Furthermore, the ancient ethnic wars in China were continuous, and the ethnic hatred caused by the war massacre was dominant. The Song Dynasty considered itself an orthodox Han regime and had to draw a clear line with foreign regimes. The rulers of the later Han and Northern Han Liu were actually Shatuo people. The Khitan nationality used to be the patron of the Northern Han Dynasty. Yelv was originally the national surname of Qidan, and later it was widely changed to Han surname, mainly to Liu surname. Of course, the Northern Song Dynasty was unhappy.
It can be inferred that according to the above reasons, how dare the author of Hundred Family Names offend the authorities? At present, the most widely recognized version of Baijia surname comes from the hand of a Confucian scholar in Qiantang, Song Dynasty. Therefore, Liu's ranking should be deliberately put behind, so Liu's ranking is very low among hundreds of surnames.