Why does enriching the mixture ratio beyond stoichiometry give more power?
in practical terms, running rich means that despite the unevenness of the fuel-air mix both in a single cylinder firing and across all cylinders firing and across all firings in the engine, all cylinders are guaranteed to NOT be running lean on ANY firing, which means all of them will be pulling best power on EVERY firing. 

In addition,  there is an effect that I know about from motorcycle engines which is probably true of airplane engines and that is the "quality" of combustion as a function of richness: If you run rich, preignition and lean misfire (both of which reduce combustion efficiency) are prevented, and the delivered power is smoother. You can hear this: a motorcycle engine sounds "happier" when running rich than it does running lean- and it pulls harder, even though the mix is nonstochiometric.