I've drawn a schematic that might help:
simulate this circuit – Schematic created using CircuitLab
The single 50 Ohms is the impedance of a single-ended transmission line that has a ground connection.
If you use two of those and operate them differentially !!! then the characteristic impedance can be treated as a 100 Ohms differential impedance. This 100 Ohm has no ground connection.
But there's a ground between R7 and R8!
Well spotted, but if the signal is purely differential then the voltage at that node will always be zero. If the Data+ = + 1 V and the Data- = -1 V then that node between R7 and R8 will be at 0 V anyway, the fact that it is connected to ground makes no difference! There is no current flowing into that grounding point, so nothing changes if we remove it resulting in R9 = 100 Ohms.
Notes: the resistors show the termination resistors that are needed to properly terminate the transmission lines. Assume that the characteristic impedance of the transmission lines is always 50 Ohms in this example.