On the left, McDonald's French fries from late 2008. On the right, KFC fries from the very same day. As you can see, the results differed quite dramatically.
One good question is why? People have suggested various things. One theory believes that McDonald's prepares their fries in much hotter oil, thereby killing all of the things that could grow mold in the future. Another suggests that the ingredients in the McDonald's French fries simply are not conducive to bacteria growth.
Atopiary himself admits the experiment is not scientific but "it creeps out anyone who looks in the window sill," he says.
Apparently those McDonald's fries are good enough for Rihanna though.