What you perceive as a shadow is in fact an area of pitch that is only lit by 3 floodlights (i.e. one is blocked by the player) instead of all 4. That's why it looks a bit darker.
now lets look at the facts, there are usually 4 lights, 1 in each corner, so they all face in different directions. So horror of horrors, they cast 4 shadows. Not too tricky is it?
well it's a bit like why there are sunspots. take those areas away from the surface of the sun and they are still increadilbly bright, just slightly less bright than the rest. Similarly there is a tiny variation in the intensitity of the light the further they are from the source. In the middle there are 4 faintshadows, in one corner ther are some faint and some less so due to where they are.