In fine print at the bottom of the page there is some information about the reference ranges. For the Hologic hip measurements, the reference range from the NHANES database is indicated by "NHA". The older reference range is noted by a "TK" and date. Most of the machines switched to the NHANES data base by about 1997.
When bone density was just starting, the databases were smaller. In those days the reference ranges were healthy individuals, usually without fractures or diseases known to be assoicated with osteoporosis. The Hologic hip reference population was particularly healthy, about 0.5 standard deviations better than the spine population.
The NHANES database was a stratified population based sample including all non-institutionalized persons. Thus, the reference ranges became lower. For example, the cut-off values for osteoporosis using the densitometer menufacturer's database yielded a prevalence of 41%, whereas the cut-off values from the NHANES database resulted in a prevalence of 20% (Looker, JBMR 1995, 10:796).