U need to understand something about writing a Story, the author can't always directly state A >> B and >= C
It will feel repetitive over time and sometimes forced, so the Author relies on different things to tell the same message
In Baratie, Gin one shot Pearl and then Gin stated no one can defeat Krieg
In Arlong Park, Arlong defeated Zoro and Sanji after they overwhelmed Hatchan and Kuroobi
In Arabasta, Officers are Ranked by Numeric System
In Skypiea, Priests are Ranked by chances of surviving their Trials
CP9 are Ranked by Doriki
And so on ... etc
So don't think much about it, Doriki is not important in the grand scheme of the story or world, it was just added in that Arc to help powerscalers judge and understand the fights and how difficult they were
As for that pattern of Zoro vs Serious No.2 and Sanji vs Goofy No. 3 only happened thrice in almost 1100 Chapters, so stop thinking too much into it
Actually Mr. 2 is lower rank than Miss Doublefinger and his Fight was followed by Nami Fight and then Zoro's
So in reality, that pattern only applied to Enies Lobby and Wano