Your example is not fair....

Stereo you will hear panning of instruments and different locations within the stereo field.. (this is the ideal sound)..

Mono will combine left and right stereo outputs together... voiding all panning and stereo imaging...(yuk)..

Your example is a stereo signal and you are removing half of the stereo field, only hearing a partial sound... I bet you sure can hear a difference..

A true mono sound you will hear these signals combined... and it will never sound as good as a true stereo..