Ah, the 'Human Observer Method'.
The cold, mist-shrouded lonliness of the Touchdown ROP.
"6 lights".
The location from which 'met vis' is measured is often a considerable distance from the runway. The observer reporting the 'met vis' is often in a different micrometerological environment (in the terminal complex/ATC Tower) to whom, or whatever, reports RVR, (out on the airfield, with less anthropogenic heat to evaporate any mist or fog).
Hence the often great difference between the 2 measurements.