Ahh, I may have misunderstood your first post.
When you talk about the speaker volume, are you meaning the ability to set different levels on the right and left channels, or just the system volume in general? If you mean the ability to set different levels on the left and right channels (and sub), the reason for this is, not everyone is able to set their speakers up so the left and right are equidistant from their sitting position. As a result, you can set different volume levels (and sometimes distance as well) for each speaker and the sub to ensure at the listening position, you get the same volume level from all the speakers. There should be a test tone which allows you to listen and then set the volume level accordingly.
If you don't mean this, then I'm not sure I understand what the issue is I'm afraid!