The lighting in a map is coded into it when it's compiled. It can't change in the .bsp. All the lighting adjustments after that are server and config settings. Possibly your config.cfg and autoexec.cfg have some settings that adjust the lighting in multiplayer. If you're building and playing the map in WON and you have to play multiplayer in STEAM, it can be dramatically different. I have a base config called "mapconfig.cfg" that I execute before I test a map. If I build a map with my regular config, the lighting is very different.
If you have any cvars in your configs that begin with "gl_" , you could be messing with your video. "gl_log" set to "on" or "1" in your config should write a log of what's happening with your video. I've never used it, though, so try it with caution.