Yeah, there's quite a bit of "bad math" out there in the specs.
My Yamaha RX-V1800 says (in the specs):
Minimum RMS Power Output for Front, Center, Surround, Surround Back... 130W
7 channels x 130 W = 910W, but they don't make the "all channels driven" claim.
Later on in the specs, it lists 500W as the power consumption for US models and 1100W as the maximum power consumption for the "General" model (whatever that is).
If we were to assume that the amplifiers are 60% efficient (and this might be overly generous), it would take over 1500 watts of input power to produce the 910 watts of output.
To the casual observer, it might seem that 500 watts of power from the AC mains can produce 910 watts of power to the speakers. If this were true, I'd buy a rack of them and power my house off the excess.
As far as I know, the receiver contains 7 identical amplifiers. The power that doesn't go into amplification is dissipated mostly as heat, and the receiver does get quite hot. I could probably assume pretty safely that any individual amplifier is capable of the 130 watt specification, but in tandem, the power supply cannot handle all 7 amps running 130 watts at the same time. And even if it could, the chassis isn't designed to handle the heat that would be generated.
So... inflated numbers and a compromise. The important thing is that it will drive the speakers without distortion and at a level that works with a bit of headroom to spare. I don't imagine commercial/pro amps are spec'd in the same way.