RVR - Runway Visual Range - 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
Visibility - The greatest distance under given weather conditions to which it is possible to see without instrumental assistance.
The important is: "RVR is calculated from....runway light intensity"
So typically High Intensity Runway or Approach Lights are more visible than unlit objects and thats why RVR is greater than visibility.
David