Volume hotkeys don't work correctly with 5.1 sound

[Tralalalala]

Today I installed a Asus Xonar D2X in my system. Changing the volume by using the scroll wheel of the mouse on the volume icon in the top bar changes the sound the way it should.

I've got a keyboard with hotkeys for changing the volume (The Tux Keyboard from Zareason). When I mouse over the volume icon in the top bar, I can see the volume is at 100%. Now I press the XF86AudioLowerVolume key and the volume is set to 25%. I press the key again and the volume is set to 23%. I can only see this when I mouse over the volume icon. Notify-OSD shows the volume is still above 90%.

Now I press the XF86AudioRaiseVolume key two times and notify-OSD shows the volume going back to 100% (it also sounds like the volume is at 100%). When I mouse over the volume icon, I can see the volume is at 26%. Now I use the scroll wheel on my mouse to raise the volume and it shows the volume going to 30%. Sounds are played less loud now. I can clearly hear the volume falling back from 100% to 30%, although I scrolled to raise the volume.

Well, it's a much easier to explain when using screenshots (see attachment):
1) The volume is at 100% for all channels;
2) I press the XF86AudioLowerVolume key on my keyboard and the volume of the front speakers are lowered by a few percent, while all other channel are being muted (mouse over on the volume icon shows the average of all channels, which is 25%);
3) Pressing the XF86AudioRaiseVolume key raises the volume of the front speakers back to 100%, while all other channels remain muted;
4) Scrolling up one time raises the average volume of all channels to 30% (the volume of the front speakers fall back from 100% to 30%, while the other channels are raised from 0% to 30%).
Now I can scroll back to 100% for all channels.