Most people dont have understanding of power level.
Yonko, commenders, Admirals, Shishibukai are just a title, they are related to strenght but its not something absolute.
If being a Shishibukai require 15 and you are 300 you are still a Shishibukai, because Shishibukai is a position. Even though it require strenght its a position.
If being a Commenders require 30 and you are 170, you Can be 1500 you are still a commender. If you are bellow 30 you aren't commenders level. You are going to get stomped by a true commender.
If Being an Admiral require 120 and you are 300 you are still an admieal why? Because its just a position.
If being a Yonko require 150 and you are 800 you are still a Yonko, why? Because Yonko is just a title. You need some power level but overal its a title.
Once why even some commender like Rayleigh who are more than qualified strenght wise to be Yonko aren't cause they dont fit the requirement for the position.
Once why being a Yonko, Shishibukai,commenders, Admiral doesnt make you officialy stronger than the others.