Apply linear superposition of complementary signals.
If they are perfect in the ideal world, the net sum = 0 This is the common mode .
But skew expects due to differences in rise time or prop delay resulting in CM pulses which can interfere with other high impedance inputs. The Fourier spectrum for a pulse is broadband except for recursive notches where f fits in the width of the pulse.
e.g. 10ns Pulse has a notch in energy at 100MHz , 200, 300 .etc and reduces in amplitude due to rise time.