From what I remember, there was some kind of light lookup table that had one entry per tile column. It was only a 1D array, so not suitable for full 2D light calculations.
When drawing the screen, I believe the visible portion had some 2D array. It was probably initialized using the 1D array, and then updated based on the light radius of units. I believe this 2D array was only really valid for the visible portion, and hence not suitable for calculations affecting game engine results. You wouldn't want to game to go out of sync because two players were looking at different areas of the map, or to have the game respond differently because you looked somewhere else.
Edit: Also, I remember something about blight growth always being affected by day/night, even if it's not enabled. That was a little strange.