If you look at the route on gcmap: (http://www.gcmap.com/mapui?P=YYZ-HKG) the direct patch goes almost over the north pole but a little westbound.

There are 2 other factors that determine the actual route. 1 is ETOPS rating where during winter months some airports are not available as an emergency alternate airport.

The second, and most influenceable is trade winds, Jetstream, Gulfstream air currents.

Generally going westbound is better, but when stronger air currents are prevalent, they may go eastbound to avoid a strong headwind, and/or benefit from a strong tailwind.

SQ's old EWR-SIN was notorious for sometimes going either way.