Some Googling around turned up the following (badly translated) information:
07.04.12 [April 2007 -hugh] 16971kHz 10kW JSC Fisheries Kagoshima consignment sent to radio stations (for the direction of Hawaii)
I interpret this to mean that this one frequency is broadcast for part of the day over a separate transmitter with a different beam. This certainly explains why only part of Kyodo's 24/7 sked is being sent, and why it's so much louder on the US Pacific Coast than any of the others. It sounds like a special contract to transmit a newspaper to Japanese fishing boats operating in the Eastern Pacific.
You learn something new every day in this hobby.