Could the rich or tired also be the case when the MC21 made 60 when the MC18 made 62? Perhaps the only reason the MC18 made slightely more is because it was fresher and slightley better jetted?

Going back to the original question..as Fontyyy said, you will probably find the MC21 is still restricted and the MC18 is not or the MC21 has an problem, both restricted or both de-restricted there is very little between them.