It's not clear why a weak yen would cause round trips originating in Japan to be less expensive than those originating outside Japan.

I thought that the weak yen should give Japanese carriers a financial advantage. But given that JAL/ANA are not discounting relative to US carriers there are only two explanations. Either JAL/ANA's costs are largely denominated in dollars in the form of gas, airplane costs, landing fees and so the weak yen's influence in muted. Or the Japanese carriers are pocketing the exchange differential. I think it's likely the former, but it is strange how origin of flight influences the price.