At least in FAA/NWS land, visibility and RVR are two different things. From the Federal Meteorological Handbook:
Prevailing visibility.
The visibility that is considered representative of visibility conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
The runway visual range is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.
So RVR is calculated for a specific runway while visibility is a general airport observation. Since the conditions at the end of the runway could be lower than the stated visibility, visibility mins are bumped up a bit to account for any variation from the airport observation. If you have a RVR report for a specific runway, it trumps the airport visibility report. If, for any reason, a RVR report for a specific runway is not available, you use the airport visibility.