KebaNYC:
The issue is that NW does not release many seats to CO for the NRT-BKK route. Hence, the CO site prices the whole itinerary as Y or J. But NW does release seats to SIN, so if you price out JFK-SIN, the prices are a lot closer, though CO is still usually about $150-150 more than NW for the same seats. The same holds tru for CO metal out of EWR to NRT and then NW beyond.