Because the earth is not strictly spherical (it "flattens" a bit due to its rotation), there's a mathematical adjustment that can be made which makes the result even more accurate, given any two locations worldwide.

(I researched this about a year ago for a web application involving searching for points of interest within a specified radius of a reference point. I found various algorithms and chose one, which I translated to VBScript. It's nice to see the SQL implementation.)