Sound volume is adjusted in opposite direction by the mouse wheel in System Settings

[The bug]
In System Settings -> Sound, the "Output volume" gauge reacts in opposite direction on the mouse wheel scrolling. That is, scrolling in the direction away from you leads to DECREASE of the volume, while it is natural to expect increase. Moreover, the sound indicator behaves well, as expected. Thus, the volume gauge in System Settings and the gauge in the sound indicator react differently on the mouse wheel scrolling.

[How to reproduce]
1. Hover the mouse pointer over the sound indicator.
2. Scroll the wheel in the direction AWAY from you. The volume INCREASES. This is correct expected behaviour.
3. Open System Settings -> Sound, hover the mouse pointer over the "Output volume" gauge.
4. Scroll the wheel in the direction AWAY from you.

[What is expected]
The volume should increase.

[What happens]
The volume DECREASES.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity-control-center 14.04.3+14.04.20140410-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-24.46-generic 3.13.9
Uname: Linux 3.13.0-24-generic x86_64
ApportVersion: 2.14.1-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Apr 27 16:14:16 2014
ExecutablePath: /usr/bin/unity-control-center
InstallationDate: Installed on 2012-05-02 (725 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: unity-control-center
UpgradeStatus: Upgraded to trusty on 2014-04-21 (5 days ago)
usr_lib_unity-control-center:
 activity-log-manager 0.9.7-0ubuntu14
 deja-dup 30.0-0ubuntu4
 gnome-control-center 1:3.6.3-0ubuntu56