Balanced impedance CM and DM stages EMI Filtering
Think of a DC voltage source with two equal resistors R1 and R2 connected at its + and - Terminals and a load resistor RL connecting R1 and R2.

The voltage source is the common mode signal (or CM noise). 

Now if you slightly change R1 or R2, the load resistor RL will get a different voltage despite the fact that you did not change the common mode DC voltage source.