Sabo is the chief of staff. Laffite is the chief of staff. Yet Shiryu is the one appearing in this color spread because he's the second strongest whether he holds more official authority or not. See? Use falsification instead of verification, you will discover a whole new world of how proper reasoning works.
First mate is first mate, period. If Zoro, or Marco, or Killer and so on were the first mates they would have been given such role in their Vivre Cards the same way Rayleigh and Beckman did. Your half working brain thinks it's figuring this out, but it actually isn't because you are simplifying what "first mate" means. The only thing you do is assume that the second strongest subordinate has to also take a rank of official hierarchy, but that's confirmed to not be the case —whether by all division commanders sharing the same rank, or Laffite having Sabo's rank in Teach's crew yet not appearing in the spread, or Katakuri being the strongest of the Sweet Generals yet never, ever addressed as having an official role above that and staying a minister just like everybody, and so on—; a failed assumption, therefore. In other words: pretty much everybody can see what you can (Zoro is second strongest and Luffy's right wing? No shit, impressive, confirmed, what an unexpected set of characters in this color spread...!), but others can read it better yet you think of yourself as the smart one here.
And again, cut the copium crap. I'm not even a Sanji fan yet you seem so oblivious to reality I bet you think I am.
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Hierarchies need to be easy yet Sabo is in here as chief of staff of the Revolutionary Army while Shiryu is here instead of Laffite, Tsuru isn't stronger than the admirals, Trébol was portrayed in the same ballpark as Diamante and Pica in comparison to Vergo, Jango was easily defeated by Usopp, Boo isn't more powerful than his grandpa and Mohji and Cabaji are virtual equals (arguably, the chief of staff may be stronger than the first mate here) and weaker than most of Buggy's new crewmates.
Power =/= hierarchy. Tons of counterexamples, and even if it truly amounted to just 1% it would still make room for any second strongest to not be the first mate as the official role it is.