Also true, at least for Sulu in the first year. It's not a case of Scotty being referred to but not shown, as in JOURNEY TO BABEL or a few other episodes. So either Scotty was promoted or Sulu was demoted? At any rate, Scotty definitely has third-in-command status by the second year.
Chekov's only turn in the seat was during THE FINAL FRONTIER, correct? So then either Scotty elected not to assume command or Chekov has exceedingly good connections now. Obviously both are lieutenant commanders at that point. Would've been nice to see Scotty take command in the film sequels on occasion, anyhow.