Why? How can the feats lie? Rinko may have been a bit stronger than Shin was when Shin killed him, but he wasn’t that much stronger. Like ‘he wasn’t send Shin flying with one blow’ stronger.
And Ringyoku and Shuma looked stronger than Riboku.
You can argue Riboku was long retired from personally fighting by the time he faced Shuma and Ringyoku, but hell Riboku looked like he hadn’t fought for years when he overpowered Shin too.