Why does the Book of Changes use 9 for Yang and 6 for Yin?

In the Book of Changes, six rules govern Yin (three Kun hexagrams plus two short horizontal lines), nine rules govern Yang (three dry hexagrams plus one horizontal line), and Yang contains Yin.

Nine numbers are the numbers in Yi-ology, and Yang nine numbers reflect that as a king with high power, the numbers used in the political system should be in harmony with the way of heaven and earth, that is, the numbers. "The Ninth Five-Year Plan" is the title of the emperor. As far as the principle of Yi-ology is concerned, the position of five hexagrams is a position of respect, so it is the "respect of the Ninth Five-Year Plan". This is the throne, the position of leader and the high position of interpersonal relationship. Going up to the sixth place, it is the location of the ancestral hall, and it is also the leisure position and rest position of the image.