USB root hubs

An interesting one me thinks, I understand to some degree about USB etc.

But I recently purchased a webcam and even more recently I purchased a USB headset.

Both of these products draw a lot of bandwith on the USB root so one can only be used at the same time.

Then I had a look in the hardware manager thing and I saw two USB Root HUBs, the one on top has all the USB harware plugged into it whilest the other is unused and empty.

My question is this, is the USB root system a hardware or software thing as I've plugged in the USB devices in all available USB ports as in I've swapped the positions of the devices round and only one USB root hub gets used.

I'd also like to point out that there are no jumper or BIOS settings available that deal with the USB ports.

http://www.theblista.com/usb.jpg