John,
    It's well worth taking the APS course, available on line at a good price
now.   In the meantime, go to avweb and find the Pelican's Perch (John
Deakin) series on engine management.
    but to answer your question, the shape of the power vs. mixture curve is
much steeper on the lean side than on the rich side.  Therefore if your
cylinders are not perfectly matched, and you are on the lean side, you feel
it as roughness.    On the rich side, the mixtures in each cylinder can be
as divergent as your "rough" setting but you won't feel it as much because
the power output of each cylinder is not that different.   This can actually
be worse for your engine, particularly in the zone just rich of peak, where
cylinder pressures are the highest.
    The short story is you need to be a little (50+) lean of peak or a LOT
(150+ or even more) rich of peak to keep cylinder pressures in a safe place
at high power settings.   The procedure you describe results in neither on
most airplanes.

Colyn