Why do eastbound transpacific flights fly at lower latitudes than their westbound counterparts?
The different routes are due to the enroute winds created by the jet stream. The following map shows the current winds over the Pacific:


(map from skyvector.com)

You can see strong winds from the West in the central Pacific region of up to 150 knots (see here for a legend on how to read wind barbs). A flight from Hong Kong to Los Angeles benefits from these winds. A flight in the opposite direction would experience this as a headwind though and will therefore try to avoid the region. You can see that the winds are much weaker in the North.

In order to organize traffic in the Pacific, a track system has been established: Pacific Organised Track System (PACOTS).


  The Pacific Organised Track System (PACOTS) is a flexible set of aircraft route tracks that primarily link commercial air transport gateways of Japan and Southeast Asia and the gateways of Hawaii and the North American West Coast, as well as those in Texas.


(skybrary.aero)

The following map shows the current PACOTS tracks:


(map from skyvector.com)

The green tracks are for westbound flights and the blue ones for eastbound flights. Due to the changes in the jet stream, these tracks are re-defined twice a day:


  The tracks are developed twice daily by the Japanese Air Traffic Management Centre (ATMC) in coordination with Oakland ARTCC. Eastbound tracks are valid from between 0500Z and 1000Z (depending on origin) to 2100Z (with specified crossing time windows for 160E) and are given a numeric designator. The opposite direction westbound tracks are valid from 1900Z to 0800Z (with specified crossing time windows for 160E) and are designated alphabetically.


(skybrary.aero)