The lighting engine is extremely buggy in dayz.

I've been working on a custom map and experienced this the hard way after placing a few lights.

You could be literally under a light source and it'll be either complete darkness or a rave party

I'm only guessing here, haven't looked at any code but I think it is because of buildings not rendering when you aren't looking at them. The walls behind you that are blocking the light disappear when you turn your back on them and the light comes through. Happened sometimes to me when I turned my back on houses with fire in them or fire barrels.