My take and its a little different. . .
For Site Ranges, this is a number a site is presumed to cover as in its radio wave propagation footprint. Owing to many things, the actual reception Range depends on atmospherics, terrain and so on. So one could "hear" a transmitter farther away or may not "hear" a transmitter closer.
For Department Ranges, the database sets this as the area of the jurisdiction. In my scanning, there is a county some 40 miles away. I can receive their transmissions quite reliably. However, strictly going by the database range, this county would be turned off as the range is 20 miles.
As noted above Global Range on the scanner plus the Department Range taken together will result in when the scanner attempts to receive the given signals.
What folks want to do is strictly user preference and depends on one's tastes.