Whilst playing, one's attention is focussed on physical actions and the necessary coordination in response to what one hears.
Whilst listening, your attention is focussed solely on sound / music.
Big difference. That energy that was "wasted" on the physical challenges of playing is thus diverted and added to the appreciation-of-detail side of things when one switches to listening.
It's no different from the differences you notice during conversation - fidgeting or allowing one's self to become distracted as opposed to paying full attention to what's being said by another.
All that said, when anyone else has played
any of my rigs through the years, he or she has
always sounded better than I, but that's got nothing to do with this perceptual phenomenon and everything to do with the fact that I can't play for shit.