screen brightness dims despite "dim screen to save power" setting disabled

[teo1978]

Since I have upgraded to 15.04 a couple of days ago, the screen brightness automatically dims after a few seconds that I don't touch the keyboard or touchpad, when running on battery.

This didn't use to happen before the upgrades on 14.04. So I've checked the settings and found this:
  "Dim screen to save power"
which was turned on (if it's a new setting, that makes sense; if this setting already existed, then there's a bug in dist-upgrade if it gets changed).

I DISABLED the setting, but the screen keeps dimming from time to time.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: powermgmt-base 1.31+nmu1
ProcVersionSignature: Ubuntu 3.19.0-25.26-generic 3.19.8-ckt2
Uname: Linux 3.19.0-25-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.2
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Aug 17 23:17:52 2015
Dependencies:

InstallationDate: Installed on 2013-10-11 (675 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: powermgmt-base
UpgradeStatus: Upgraded to vivid on 2015-08-15 (2 days ago)