According to the graph in http://wiki.blender.org/index.php/Doc%3A2.6/Manual/Render/Cycles/Passes
adding the direct and indirect light passes together and then multiplying by the color should give the
rendered color. This is not the case in the overlapping areas, as the color also contains a coverage falloff.
Attached is a blend demonstrating the problem (switch between compositing on and off to see).
till then, David.