Die Formel geht von einer idealen sphärischen Kugel aus, der Radius ist auch "umstritten" und wird verschiedentlich anders gesetzt, daher sind kleine Abweichungen normal.
Zitat:
Either formula is only an approximation when applied to the Earth, which is not a perfect sphere: the "Earth radius" R varies from 6356.752 km at the poles to 6378.137 km at the equator. More importantly, the radius of curvature of a north-south line on the earth's surface is 1% greater at the poles (≈6399.594 km) than at the equator (≈6335.439 km)— so the haversine formula and law of cosines can't be guaranteed correct to better than 0.5%.