I will be no help here, but I have seen the exact same thing. A friend of mine made some arrows for a bow I made him, one of the arrows flew away from the gRoyo every time. We measured spine on both sides, balance point, weight, diameter, Everything that was measurable, and the arrows were about as identical as you can get, but the one wouldn't fly right. My theory based on no scientific evidence is that although spine the same, they may not bend at the same rate, but just bend to the same point when weight it applied. So when the spine weight it applied, one arrow deflects a half inch and takes ax amount of time, while the other arrow deflects a half in but does it in y amount of time. Just a theory tho........