Brightness on my iphone 5s sometimes gets darker?

The auto brightness is turned OFF. The brightness sometimes gets a little darker and then after a while goes back to normal.why? And in the settings it always says it's turned on all the way.

Mine is annoying it's always bright

Your phone is probably about to die, so it dims the display to save battery, you lock it, then unlock and it could possibly have boosted the charge enough to put it back over 20% mark. Or your eyes are stupid.

The screen will get bright and low depending on how much light is in the room

It happened to me when I was on iOS 6.1.2 on my iPhone 5 and iPod touch 5. I turned the screen on and it on minimal brightness. I turn the screen on and off and that was the fix for me. When I was a JB their was a fix for it on Cydia so it must be a software glitch that Apple needs to fix.
Now I'm not JB and my iPhone and iPod touch work fine/ don't have this problem.