According to the One Punch Man wikia the first rank assigned in screen to King was #6 but he currently is #7. Why was he demoted?
Metal Knight and King switched ranks after the meteor event. Metal Knight was promoted based on his "assistance" in destroying that meteor. Public perception, contributions to the association, and a known list of accomplishments all factor into ranking decisions in the association. It's not simply a matter of who is more powerful. In this case, Saitama's contribution to the meteor event was undervalued, and the contribution of Genos and Metal Knight was overvalued (and both were promoted).