Diffusion between potium permanganate and distilled water: experiment?

I did an experiment which was putting a sprinkle of KMnO4 into a Petri dish of distilled water. The idea was to place the sprinkle in the middle of the dish and measure how far the KMnO4 moves out during a 20 second time span. We repeated the experiment a few times with a cap placed over the petri dish, and a few times without a cap. With the cap off, the KMnO4 moved faster than when the cap was placed on.What i want to know is, where's the catch here? is it limiting oxygen? or does it have anything to do with pressure?