Jim, if you were still plugged into both the audio outs and just turned off the amp, the result would be, as expected, just a single channel coming out of one side of the keyboard. In order for the keyboard to sum both channels into the mono channel, you must unplug the Right channel plug. What your audience was hearing was just the right channel of the keyboard and no left sounds would be coming out at all if the plug remained in place. It's the way it is wired and the jack switching configuration. Try doing the same comparison again, but instead unplug the Right channel and allow the sounds to come out of the Left/Mono into a single amp.

Good luck, and I'm glad you liked that piano,

Gary