Given the absence of a highly detailed model, in this case a Charlie Chaplain mask, the illusory effects may be due to the absence of sufficient visual depth cues - similar to the effects seen in the "Spinning Dancer" effect. As a further example, next time you see a plane in the air making a turn at a great distance from you, there are actually two ways you could perceive its direction - all dependant on what you convince your brain it to be first. I would not draw too many conclusions from such a vague study.