Map Measuration Issues

I thing my measuration codes aren't precision. From Japen to China only 1300 KM, actually is 2200KM or more. How can fix this ?

/* store the location */
var loc:Location = NeoMap.map.pointLocation(new Point( map.mouseX, map.mouseY /* e.localX, e.localY */),map);
_locationPointsArray.push(loc);

_distance = Distance.approxDistance(_locationPointsArray.pop(), _locationPointsArray.pop(), Distance.R_MILES);