I remember working at Edinburgh and from the tower we could see for miles. However, there was a bank of fog/mist over the runway only a few feet thick. From overhead, pilots reported being able to see the whole runway as they were looking down on it. However, when they got to short final they were looking along the fog bank i.e. it was a lot thicker, and they lost site of the runway.
Hence RVR can be a lot less than met visibility.