Because that Air/Fuel guage is not accurate. It is only based upon the signal that is given by the narrowband O2 that you have hooked up. It is not giving you a full "tuning" base to know whether or not you're rich or lean. It's no different than a christmas ornament. Your best bet would be to get an EGT (Exhaust Gas Temperature) guage to give you a more accurate reading of what's going on...