I have flown YYZ - HKG and back several times and the route differences can be amazing. The "normal" route is up north then west over the Canadian arctic and down over eastern Russia and China, but I have seen differences as extreme as north east over northern Greenland over the high arctic and down over central Russia on one flight. On that flight there was a CX freighter pilot sitting next to me and he said that CX uses very sophisticated weather routing software to determine the most fuel efficient route for each flight.