I was going to post about how this route is not simply a great circle as it has a deviation to cut down on overflight fees generated by flying across Canada. Then I saw MarkFlies post above and realised he'd beat me to it.

Flights from SFO, LAX, etc. will often take a more easterly track to reduce the total cost. The airlines have sophisticated software tools that allow them to plug in all the various parameters (flight time, fuel burn, jetstream/weather conditions, overflight costs, ...) in order to map out an optimal route according to the anticipated conditions for the flight.

This video from Wendover Productions has more info on this (starting from around 4:25):