Twice the odd mode impedance is equal to the differential impedance so a resistor of that value is all you might think necessary for termination by analogy with simple transmission lines. Simple differential termination with common mode stimuli These real-world effects each introduce a common mode component which goes undamped because the even mode of the line is unterminated. The clean output is proof of the robustness of differential signaling, but a good portion of the receiver's finite common mode voltage range has been used up. Same as in 2 but with both modes terminated Of greater importance is that undamped signals can be driven to resonance and ultimately out of the common mode range of the receiver. To see this more clearly, we again unterminate the common mode and drive both signals in the same direction.
Inequalities crop up in line impedance or loading.
Consequently a portion of any common mode signal will be converted to differential. You can see that a fraction of the common mode signal is showing up in the signal difference. The quicker common mode signals are damped out the less will be converted to differential. Perfect termination isn't required to prevent resonance - all that is required is to make sure that more energy is dissipated per cycle than is received from a common mode stimulus. Mode conversion works in the other direction as well.