So I am no expert, but I just hit upon something strange.  
I separated out a tight 16th note arppegio pattern on a synth into Left and Right channels.   I had them panned 20% left and right, and it still sounded tight, but when I panned them hard left, hard right, the timing seems to be way off!!   like they are playing late!
 
can anyone explain this strange phenomenon?

I can tell you how to possibly test to see if it's an illusion or not.
Render the midi to audio and do the same panning.