Well, it's all a bit gash anyway. Even the best fmc model can only work to the forecast winds inserted. Assuming someone remembered to insert some! Anti-ice, inversions, turbulence speeds, weight differences, rough/smooth handling, bank angles, performance index of a particular aircraft, luck, cunning, skill, daring, finese, grit, determination, sheer rugged good looks (hostie defined) etc... can all effect ones profile.