Illuminance is basically the amount of light falling on a surface, not the amount coming from it (that's called 'luminance'). The visualization on the window is the amount of light hitting the window plane. Normally of course this is not so visible in the physical space, as that light mostly doesn't reflect off the glass, it goes through, but the results are correct. This is a common misunderstanding, as people are used to Radiance results, which override the illuminance on glazing and shows the glass as transparent. IMO it's still fairly useful information, as it tells you something about potential reflections from the glass.
David
