Weird audio balance issue

On my Mac Stdio, when using speakers connected to the headphone port, everything is as it should be.

However, using speakers that grab the audio off the USB/Thunderbolt ports, the balance control on the Sound setting panel becomes useless. The sound is definitely in stereo, so that's fine, but when I adjust the balance slider, the following happens:

Sliding from Center to Left, absolutely nothing happens.

Sliding from Centre to Right, the volume on both speakers goes down.

Very puzzling