The motivation of flying a 777 to ICN is to avoid the fee to park at NRT overnight and you get some utilization for an idle plane? But it's more efficient if you can send all longhaul aircraft back over the pacific. The ICN (and BKK and SIN) flights arrive a bit earlier at NRT than most arriving transpacs so it allows for earlier departures back to the US, and then the later NRT arrivals continue to ICN, BKK and SIN.

One factor is that they cancelled HKG-NRT Oct 26 so they have extra 737 capacity.