A cable is a mechanical system that needs burn-in. Every material subject to an electrical field creates a dipole. For non conducting material this dipole is at the atom level so that electrons are more to the side of nucleus that randomly around. The overall charge of the material remains neutral. The electrons having a mass (very very light), moving them around amounts to a mechanical action.
The atoms in the cable insulation get organized in a dipole one way or the other, when ever there is signal traveling in the wires. As the burn in progress the atoms dipoles get organized from a random state, so that less energy is extracted from the signal and more of it reaches the next component in the chain.