Turns out with some experiments and a look into the geometry files, the density is in grown hairs per square display unit, independent of the polygon density. So, if your Preferences set the display units to feet, it's hairs per square foot; centimeters, then hairs per square centimeter. What is actually stored in the file is hairs per square Poser Unit, i.e., per (8.6 feet)**2. Rather a lot, usually. [17,000,000 for average human hair.] The total number of hairs value is correctly given and is the density multiplied by the area of the hair group, which, unfortunately, is not shown in the UI, but is contained in the file in square Poser Units, however, it does not include guide hairs, only grown hairs, so the actual number of hairs is larger. For a fine mesh and low hair density, the guide hairs can actually overwhelm the grown hairs. [Been there, done that.] At least that part of the Hair Room now makes sense to me.

Whichway

P.S. - If you change the Preferences display units on the fly, you probably have to at least close the document and reopen it to get the density calculation updated.