Airline pricing is very complex. There are 11 different fare classes that are all considered economy fares for your international flight: YBMHQVWSTKL. Each of these fare classes have different prices and different restrictions.... restrictions can include (but are not limited to):

Day of week
Minimum stay
Maximum stay
Saturday night stay required

There are also capacity controls at each fare level. Perhaps the flights on the day you are traveling are sold out of the lowest fares and only higher fares are available (this is the most likely answer)...but the flights the day before still have the discounted fare available.