Ah, you are right @LiamRuvio .
It’s an overcast sky in both Legacy and in the LBT plugin. And the overcast sky is ever so slightly different from the uniform sky in that it’s a little brighter on the top than on the sides:
My point about DF not being sensitive to orientation (N, E, S, W) still holds but I guess this means DF accounts for skylights letting in more light per area than vertical windows.