Decreased Solar Gain From Shorter Shades

[mikehill85]

I am trying to analyse the effect of different shade length on solar gain in a building. The building is rectangular and oriented length-wise along an east-west axis. The vast majority of the windows are on the north and south elevations of the building (around 95+% of window area). When I shortened the shades (they are horizontal shades parallel with the ground) I increased the solar gain in every month except December. In this month solar gain decreased and I can't think why this would be. Does Apache take into account light reflected off of shading surfaces into the building?

P.S. I ran SunCast to calculate the shading effect before running Apache.