He understands Negri's point six years in, when on a trip to Komarr, the kidnappers shrug over him and take the Admirals and Generals away for questioning and leave Simon alone in the cell.
Back home, Simon mostly wears civvies. Avoiding rank tabs makes it easier. Aral keeps offering him any rank he pleases, has given him a paper promotion to vice-admiral so he's drawing a Chief's salary, but he is still Captain Simon Illyan. He still holds the rank he had when Negri died. He still holds the rank Negri had given him with his own hands.
He'd thought it was Negri not giving a damn about rank. He hadn't realized it was about being underestimated. Being ignored. Being the one left behind in a cell to break out of the restraints -- amateurs -- and deal with the problem.
This is the perfect position for a spy. Ranked high enough to not get laughed at, ranked low enough to not get noticed. The occasionally-awkward confusion is worth it for times like this, when being ignored is preferable, when being underestimated is key, when being the lone spy in the dark is the ideal.
All spymasters should be Captains.