Restricted airflow reduces sensable and latent heat removal. The compressor load goes down, as does the blower load (slightly) and the system does use fewer watts, so less power is being used. However when the system operates below is't 100% design point, the efficiency of the system goes down, fewer BTUs but BUT/KW goes up somewhat. The loss of efficiency is greater with a piston expansion valve than a TXV.
One fact to remember is that any AC induction motor has its maximum efficiency at 100% load. As the load on the motor drops, efficiency drops as well. This is true for both the compressor and blower motor. This is one way a VF blower helps SEER since the loss of efficiency in a VF motor is lower since the speed is reduced to match the demand.