I've been having the same problem on my laptop over the past couple of weeks. I haven't done any Windows updates on this laptop...as they are controlled by WSUS and I'm the WSUS admin at my office.
I'm not upgrading any of my display
drivers either. I've been unable to put a finger on what is causing this. It's noticeable when I get to work and I put my laptop into my dock and my laptop display is MUCH dimmer than my second monitor.
Edit: Yes I know this is an old thread, but it describes the issue that I am having perfectly.