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