Because an inverter/frequency driven compressor is not producing more air than needed. A conventional compressor works within a pressure band. When reaching the higher pressure the machine goes into unload mode (the motor is running but no air is produced). When reaching the lower pressure value the compressor starts to build up pressure again until it reaches its unload pressure again. An inverter/frequency driven compressor has less unload time and works towards a set pressure value, this makes the inverter compressors in general 30% more energy efficient than a conventional load/unload compressor.