Joe Plumber cannot tell WHY he hears the difference, but he knows 'something' is wrong, or better, or worse.

Until he becomes Joe Sixpack, that is...

Look, it's a well discussed issue even here at SZ, not exactly brimming with 'golden ear' studio types (of which I am not one, either ). The sound of the Yamaha pianos changes quite considerable between mono and stereo. It gets 'pinched', or 'hollow', or to the more knowledgeable, 'phase-y'.

You don't need a degree in acoustics to HEAR it.

But you might need one to 'excuse' it away...