As far as I've had it explained to me the factory ecu works as follows from the engine speed input and the air flow meter input the ECU calculates a pulse width that should give a stochiometric air:fuel ratio ie 14.7:1. The with that speed and air flow information it looks up a table which contains a number. That number is a multiplier for the pulse width. I do not know for certain, but I don't think that the multiplier can be less than one, ie acts to shorted the pulse width with should give an leaner mixture then 14.7:1.
If you are running in closed loop mode, the ECU can run leaner, but anything over about 30% load/throttle and the ECU will not enter closed loop mode, ie the 02 sensor isn't even looked at most of the time.