You are measuring it correctly. It's only the voltage across the IC that matters.
Be very careful not to confuse "gnd" or "ground" which is local to the circuit and is the negative side of the supply with "Earth" which is an assumed zero voltage benath your feet.
True "Earth" whether from a mains supply Earth wire or from a rod in the ground plays no part in this circuits operation whatsoever and it should not be conected to any part at all. The circuit 'floats' across the neutral and live wires and has no outside reference to anything else. In fact, as it operates on AC, you could swap the live and neutral wires and it would perform exactly the same.
Brian.