JHopp217:Why if the other person quits does YOUR completion pct go down?
That''s because it's a completion percentage, not a quitting indicator.
It will diminish whenever you don't play to the end, be it by quitting, disconnection, program errors or if you fail to go to the game menu or "new game" after the game.
When this was a big discussion subject after the installation, they finally excluded A/S games from this procedure. It may have changed again, but details haven't been discussed lately.