I just read the specs in the VSX-21 manual (which is the same manual for my VSX-23), and it doesn't say that the maximum power consumption is 400W, just the "power consumption." The vast majority of the time that most people use these, the amount of power being sent to the speakers to generate sound is incredibly small. Perhaps on the order of 1W total. As you turn up the volume, the power demands increase a lot - for an increase of 10dB in sound (considered by a lot of people to be twice as loud), it requires 10 times as much power. So, you do want a good reserve of power to handle dynamics and when you want to have it louder, but that's not so much my point. I think that Pioneer wasn't being untruthful in their power consumption report.
To give some perspective, here are some numbers for my system. With my VSX-23 on and several other components in standby mode, my system is drawing a little under 100W. Watching a blu-ray on a 50" plasma with an SVS Ultra, a VSX-23, and two external power amps, my systems looks to be drawing about 500W max (and that's with bright scenes on the plasma).
Just some thoughts.