[ellisv]

This is similar to the difference between WGS84 and NAD83 (North American Datum 1983). WGS84 is defined by the average of stations world wide while NAD83 is defined in a way that it is relatively constant across North America. NAD83 initially matched WGS84 but their difference increases each year by a small amount (negligible for most consumer use). Of course there are other differences between NAD83 and WGS84 as well (reference ellipsoids) and often they are treated as the same by non-cartographers.