Izi's explanation about shadows on the blinds is correct. When the light is high enough, the blinds appear to overlap and so the whole square blocks light.

But here, with the light coming from a lower angle, it can go between the blinds.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)