The low load factors are widely known about SEA-HND, there is data out there showing that there were months last winter that were in the 30-50% range.

Most tickets are not booked in advance.
80% of domestic tickets are booked within 5 weeks of departure. Leisure and international may be a little be lower amount. Business heavy routes will see even a greater percentage.
This trend has amplied in recent years and makes it all the more challenging for airlines to perform capacity management and planning. There are certain lead times where they schedule has to set before the majority of the tickets have even been purchased.