there are some weird differences between winamp on different computers.
On my computer, winamp's volume is entirely independent of either my system main volume or my system's wave volume. I've seen people's computers where it is hooked directly into the wave volume so that as you change the volume in winamp it changes it on the mixer in real time, and other where winamp changes the system volume but doesn't move the mixer slide bar. I'm sure this is both winamp version and sound driver dependent.
That probably isn't very helpful, but go open up winamp and see if when you move the sound around in there if the mixer's wave bar moves with it in real time.