There are any number of reasons why ones electric bill could be lower one month over the next. Its getting warmer out so there is no need to run the heater. Could have taken less showers so the water heater didn't work as frequently as before. Because its nicer out he could have been outside more thus the electronic's in his house weren't running as much as last month. He could have recently switched the bulbs in his house to low wattage, high efficiency CFL's. Any number of things can explain the power saving and I can, with significant reasoning ability say, that this plug and save isnt one of them.
There is no end all be all power saving device. I say again, THERE IS NO END ALL BE ALL POWER SAVING DEVICE.
If there was such a device to conserve and redistrubute power on demand every home in every town in every state would be using it.