The K0--K0-bar, D0--D0-bar, B0--B0-bar, Bs0--Bs0-bar systems all exhibit oscillations whose rate is proportional to their mass difference via a second order weak interaction "box" diagram. I don't understand how their masses can differ, when they are simply C conjugates of one another. Doesn't the TCP theorem forbid a difference between the masses of a particle and respective anti-particle? I assume the mass difference between a K0 and a K0-bar (or in the other systems) is caused not by a difference in the masses of s and s-bar or d and d-bar, but by something else. Can someone explain how this mass difference comes about?