No, it has nothing to do with auto brightness. It is a built in feature of Android which aims to save energy, but sometimes ruins the picture quality. You can search in Google what CAB is, for detailed explanation.
When I had a Nexus I use to change the acl_set file in /sys/class/backlight/s6e8aa0 , but it is not the same with this device. So I hope someone knows how to disable it on the Optimus G.
Samsung Galaxy S - > Samsung Galaxy Nexus - > HTC one X - > LG oPossum G