Apologies if it’s obvious but I always hear that differential pairs have a negative and positive waveform (one on each line) that’s opposite. Now i get that you could choose whatever value or point “Ground” is and just have the pulses swing around that point. And its that reason why the grounds between two diff pairs does not matter since its just the difference that matters . But I struggle to see how the negative line could be at a voltage negative with respect the ground reference used to make that actual signal. Say for example on usb, is the negative line of the pair actually below the GND line of the usb? Or is it a case of the negative of the diff pair could be 0-2V and the positive line 2-4V (with respect to the actual usb ground)? 

Hope my question makes sense