Why is there a disparity in the conversion from runway visual range (RVR) to statute miles?
Some light on the mysteries of conversion...
Types of devices used for RVR measurement
A RVR device is composed of a light source and a light sensor. The light source is an incandescent lamp in a transmissometer, the oldest device, while in the more modern scatter-effect device it is an infrared LED. The sensor can determine RVR in a certain range, with the accuracy varying in the range:

Incandescence: Down to 600 ft by 200 ft increments within 600-3,000 ft, 500 ft above 3,000 ft.

Infrared: Down to 0 ft by 100 ft increments below 800 ft, 200 ft within 800-3,000 ft, and 500 ft above 3,000 ft.


RVR unit conversion
To make RVR ft-m conversion easy, rough equivalences are established:

100 ft = 25 m
200 ft = 50 m
500 ft = 150 m

Visibility unit conversion
Countries can also report visibility, which is different from RVR, in statute miles or nautical miles. Again rough equivalences are in place:

1/4 SM = 400 m
1 SM = 1,600 m.
1 NM = 11/4 SM.
1 NM = 2,000 m.

Substituting visibility to RVR
With all these approximate conversion factors defined, we can build a table for visibility by increments of 1/4 SM or 400 m (on the right hand of the table below), and use meter values, the common unit, to establish approximated RVR-visibility pairs.

(Source: Jeppesen)
Visibility, contrary to RVR, is not an exact measure. It's based on eye performance and available distinctive features of the surrounding environment. Moreover, visibility isn't generally evaluated for the same volume of atmosphere than RVR.
Sources:

FAA Order 6560.10C, Runway Visual Range.

Jeppesen Explanation of Common Airport Operating Minimum Specifications