Do you have your function keys set as regular function keys or special function keys? As special, an F2 key press adjusts brightness higher if you inadvertently tap it.
If that's not the issue, another one is that your MBP will reset to default Power Management brightness at re-boot. It should not do this unless you want it to, and to clear the behavior you can to do a Prom Reset --- hold the P and R keys down as you boot, holding them until you hear the SECOND ding, then release. All your settings should be reset now, though sometimes you need to repeat this process twice to have it take. Now set brightness to desired level and restart, and make sure your original settings stuck through the restart.
Cheers,