The RA antennae are located on belly of plane.
The readings will be 0 for an inflight pitch attitude during flare, hence 0 ft is read at touchdown, negative values displayed when on ground.
At higher altitudes, due to the high variableness when flying over non flat terrain, such trivial differences in radio altitude can and thus are ignored.
But when over the landing strip, in LVO conditions, the aircraft must know its actual altitude above the ground and thus relies solely on the RA, as does the auto throttle FLARE command use its input and the trim change occurring prior to landing.
The slight difference in RA values displayed in the flight deck is due to the two antennae not being collocated nor shared.