Originally Posted by
skatefriday
It's not clear why a weak yen would cause round trips originating in Japan to be less expensive than those originating outside Japan.
As others have pointed out, many variables play a role in air fares, of which relative currency strength is only one. But, if you look at the mileage run deals over the course of the past 20 years, it's impossible to overlook the fact that some points of origin pop up over and over again for 6+ month stretches, and these are often places with weak currencies and/or economies.