i suppose it is because movers can "move" and the engine can't lit objects in real time, so the mover gets lit by all the lights on the map that can reach him, but without being stoppped by obstacles.
if you wan't to make it darker, then you should try to lower the value of the "scaleglow" setting inside the display menu of that mover's properties. set it from 1 to 0.5, and rebuild the light and see if it's better.