Originally Posted by
limabean767
SFO/LAX/SEA/YVR are a bit different because the majority of the flight can be flown outside the jetstream, or a shorter distance (over greenland/northern canada) on the way out avoiding the jetstream, but on the way back taking advantage of the jetstream, but on a longer route (northern USA, normal north atlantic route), hence the time difference is not so large.
I don't know about CUN but certainly flights to MEX often still deviate way North to find the most efficient route ... I remember being on an LHR-MEX flight which went over Iceland, central Greenland, Hudson Bay and then pretty much due south over the US to MEX ... but then on the MEX-LHR the route back was much more straight line looking passing out over Florida and straight over the mid Atlantic to LHR.
All sorts of complex calculations I'm sure go into determining the optimum routes .. it often seems to me that I'd prefer the shorter flight to be the outbound day flight rather than the night flight home .. but I guess you can't argue with the winds !