It's what I've been saying all along. In addition to just being a better amp in general (the 2200/200 is just not that great), the very much larger output transformer of the multichannel amps can dump a lot more current into any given channel when they're all not being driven to max levels simultaneously (which probably rarely happens except on the test bench). So if you're listening to stereo programming the tranformer is sitting on a lot of excess capacity that it can bring to bear on the 2 channels actually being driven.

Cheers,