This is a very good point - I don't know why I didn't think of it before .

There really is no universal standard to measuring Great Circle distances - I can pretty much guarantee all airlines/FFP's will come up with different figures for the same route. It may well be that A3's is actually closest to reality, or someone's else, or maybe everyone's is equally far out - we just have absolutely no way of telling. That's why, as the OP said, it simply doesn't pay to rely on any mapping tool/website, ever, if future status absolutely depends on accuracy.