It's only a mistake fare if you find a good deal to TYO. It usually costs 1/2 the price to fly twice as far and connect in TYO, and their TPAC flights to SE Asia have been very affordable for at least the past 3 years. The odd thing about NH is that fares usually seem to be 50% less when booked codeshare on UA. The prices also seem to vary widely depending on exactly how you organize your search on multi-trip flights, even when looking at the exact same flights.