Watch Dogs had five days and it hasn't been released in Japan. (actually I'm not sure if their PR was based on the tracking weeks or a full week, but VGC data will be five days.) Mario Kart had 4 days in Japan.
Also if there had been a PC version of MK8 that sold 50k would that somehow make MK8 sales worse since it brings the average down?
Well if MK8 hade launched for PC people would have pirated the game and the WIi-U version would have sold less. Nut i don't think that was what you meant .