with armored cable with a bonding strip or a grounded box plus an outlet that allows grounding through the strap. On the contrary, properly grounded AC cable is very common.Whether or not the yoke provides an adequate ground, depends completely on the device.Post a picture of inside the box and the receptacle.
I'm even more confused now that the receptacle tester said that they were correct. Are there ground wires in the box (you'll have to remove the cover plate, and possibly the receptacle to see)?
Is there a brass colored tab on the yoke where the mounting screw attaches the receptacle to the box (you'll have to remove the cover plate to see)?