I think @krollo is talking about the default ambient light that most cells have when there's no light objects present. I've been wondering the same thing, because I still have lots of ambient light and default fog in my custom interior cells. No matter how dark it seems in the CK view, it's never that dark in game.
I expect it's a matter of setting the Lighting Template correctly, but I haven't been able to find any guidance about that, with regard to making things really dark.
I quote radiumskull he explain the problem better, my interior cell is new, and doesn't matter if you put light in the cell because it has a default ambient "super" light, its like it is illuminated by the sun.
On the CK i edit the interior cell, clik lighting and doesn't matter what template or fog i chose because doesn't change anything, a bunker without windows and light its still magically illuminated.
Edited by krollo, 14 July 2016 - 04:15 PM.