"When an amplifier is pushed to create a signal with more power than its power supply can produce, it will amplify the signal only up to its maximum capacity, at which point the signal can be amplified no further. As the signal simply "cuts" or "clips" at the maximum capacity of the amplifier, the signal is said to be "clipping". The extra signal which is beyond the capability of the amplifier is simply cut off, resulting in a sine wave becoming a distorted square-wave-type waveform.
Amplifiers have voltage, current and thermal limits. Clipping may occur due to limitations in the power supply or the output stage. Some amplifiers are able to deliver peak power without clipping for short durations before energy stored in the power supply is depleted or the amplifier begins to overheat."
With multichannel amps each channel draws what it needs form the power supply. When the total drawn by all the channels reaches the maximum capacity of the power supply you get clipping. There is no "allocation" of power.
Cheap AVRs will have an inexpensive power supply which may be able to run 2 channels to a decent level but will be unable to run all channels at that same level.
More expensive AVRs (Arcam) will have bigger power supply which will enable all channels to run at a high power level simultaneously. They will produce even more power per channel if only running 2 channels (subject to output stage and thermal limitations).
Naturally cheap AVRs specs won't quote power with all channels driven as it will be a low figure. They might also quote power into 6 ohms instead of 8 ohms, at just 1kHz instead of 20-20kHz, and with high or unspecified distortion levels. Not exactly BS but not very informative.
You can use the max power consumption as a rough guide to the capacity of the AVR's power supply, keeping in mind that different amp classes have inherently different efficiencies (i.e class A/B 60-70% efficiency, class D over 90%) and hence different power supply requirements.