The jetstream / winds across the pacific in winter make for much longer Asia bound flights and consequently shorter USA bound flights (owing to even tech stops some times at ICN/TPE etc at times on the LAX-HKG flight, but much less since the 77W came into play).
In the summer, these winds are less active and hence shorter flights Asia bound and longer flights USA bound.
The reason they change with summer schedule is because CX or any airline couldn't publish block times every week - so they operate two sets of block times. But you will find a LAX-HKG operating much longer in Dec/Jan/Feb and a HKG-LAX much shorter in that period. In the months leading up to and after this winter period the times equalize as the season changes. For example in winter I have had a LAX-HKG take 15hr50 mins, and in summer I've done that flight in about 13hr45 mins.

JFK and ORD are less influenced by this due to the Polar route.