The radio altimeter is calibrated to read main wheel height above the ground while in the pitch attitude associated with a normal configuration and normal approach speed at the runway threshold. In most large transports, this means that the radio altimeter antenna, located forward of the main wheels on the fuselage, is quite a bit higher than the main wheels. When the main wheels touchdown, the radio altimeter should read zero, but the antenna is still several feet above the ground. As the airplane de-rotates and the nose touches down, the radio altimeter antenna continues to descend, and finally thinks it has gone below the ground level.
On the 767/757, the radio altimeter typically reads -8 feet on the ground, although the DH selector knob will rotate as far as -20 feet.