Because lowering the speed does not stop cooling in an inverter rather it adds just as much cooling as is wasted by occupants breath, window and door heat leak-ins from outside. Result is temp remains near to constant with only slight variation. At this stage AC is only conauming 350 to 500 watts depending on Heat leak in or cooling loss. (Room size, thermals etc)
2nd place where inverter compressor saves current is it does not slap the power line on the back of its neck everytime it starts like its counterpart does.
This controllable speed on rotation of compressor that the inverter offers also assures longer compressor life because most of wear n tear occurs each time compressor kicks in...
All this creates a difference of 40% less energy usage. Why? Think again ... Non inverter takes 9 amps everytime it goes on.. Inverter takes 9 amps only for 1st 20 mins.. then all night its between 1.8 to 2.2 amps 80% of the time.