How are night textures handled in FSX / P3D?

I was attempting to tweak some dds texture files on some airport buildings today, and the results were unexpected. I had assumed that the _lm dds textures were simply replacement textures for the daytime ones, but it seems that some kind of blending is going on between day and night textures, that results in a brighter highlights. Is this correct? As a test I used the same texture for a yellow window glow for both night and day, and the night version was much lighter and harsher than the day version which seemed dull in comparison. I'm trying to find an explanation for this if anyone can help.

Jason