What survives subtraction?

A toy receiver subtracts B from A. Both lines receive the same added disturbance unless you introduce a mismatch.

The wanted voltages are +0.20 V and −0.20 V. A balanced interface is defined by impedance relationships, not a requirement that all wanted signals be equal and opposite. This toy calculates voltages only; it does not simulate cable impedance, grounding, or real receiver limits.