I don’t see anywhere in my post I mention having seams between modular pieces. It has nothing to do with modularity and that link (which [MENTION=4894]Tim Hobson[/MENTION] also posted).
It has to do with floor receiving Sky Light (my latest theory at this time). I bet if I shrink the model of the floor to be enclosed inside the room, it will be lit properly. However since it sticks out (part of it inside the room, part outside), it ignores the indoor factor.