The Total power is limited by the power supply and the rest is decided on by the design of the system. As I mentioned the Sony has 710W available but at 10% THD to all 5 channels, this would sound terrible so a realistic listing might be 10-35W/Ch(just a guess) and at this level only a small room with lower volumes would work well with this receiver.
This is where that minimum spec listing comes into play. Manufacturers are ALLOWED to list there specs as one channel driven and the rest at 1/8 power. It takes a much smaller P/S and amp to do this. It is from this spec that many low end avrs are able to get such high Watt/ch rattings, because they in fact can achieve the specified rating on any of the channels just not all at the same time.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70