"we have to lean the mixture on that mag "
OK, what you are doing is leaning the mixture on that set of spark plugs not on that mag
This is an attempt to clear debris on a spark plug(s), by running a higher EGT than would normally be the case on the ground.
What I am curious about is this: as I well know from my IO-540 experience, a bunged-up plug makes the engine (and the whole aircraft) vibrate like hell during the mag checks, and one doesn't need to look at the RPM to realise that one has a duff plug. (I can also see it on the EDM700 which shows a zero EGT at that time on that particular cylinder).
Is it possible to have a partially bunged-up plug, just enough to cause say an extra 50rpm mag drop?