Optimum engine controls depends on all sorts of things. Running the air-fuel ratio at stoichiometry is not likely to result in the best fuel economy (emissions ignored). Typically to get the most fuel burned (and the most work out of the fuel you put into the cylinder, you have to have a little extra oxygen to make sure that (almost) no fuel molecules can't find some oxident. Similarily, to get the most power one would add extra fuel so that all the air (typically the limiting factor) gets used.
Over fueling and suboptimal ignition timing can definately hurt fuel economy.