Why is the soil moisture variable in the Reanalysis Dataset dimensionless?

My understanding from the Reanalysis documentation is that the soil moisture represents the fraction of the soil column (between 10 and 200 cm) composed of water, by volume.

Please see the following article in the Bulletin of the American Meteorological Society for more information:

http://wesley.wwb.noaa.gov/bams96.html

--Michael Bell