You're gonna see weirdness like this because they use a trick to render shadows and not a true raytrace engine. Basically they render the scene from the light sorce to create a map of what is shadowed and what is not. Then, as the render the scene they either reverse project the pixel from the shadow map into the scene to determine obscurity or they render the shadow map as a polygon with an alpha on it right to the scene. In either case, the results will be inaccurate to say the least. What they are not doing is what higher end packages like 3d studio do when rendering shadows.