Daylight factor issues


I have a strange issue with my daylight factor calculations. I am trying to check the daylight in an atrium with a glass roof. For the calculation I use zones from Archicad with the outlines of windows projected onto the zones in Rhino. This I add in Grasshopper as rooms with subface and bordershade. The glass roof is added as an aperture directly to the HB Model. My problem occurs when I change the transmission for my glass roof. A transmission of below 0.4 means total darkness, and above 0.4 is daylight bonanza. I would expect a more gradual rise in my daylight factor and not this extreme difference just by going from 0.4 to 0.41.