Yeah, I'm a little confused on this too.
Best as I can tell, the stock program only allows you to set a certain level of lowest possible brightness. Some users find even the lowest setting to be a bit too high - consumes too much battery and is just needlessly bright. Once rooted, one is able to set the minimum brightness to below the stock minimum. I believe it is actually - 9%. This is done by selecting a setting thru Newly created "bright control" icon.
I download a free app called "fast bright" in market. you can select a certain level brightness and with a widget, one touch to get the screen really bright. I find the widget useful in direct sunlight and have it right on the home screen.
But, I digress.... Yes, you're right, the whole thing is confusing 'cause it seems to just randomly switch.