I've admittedly only been looking at MEL-OOL on Tiger at the moment, and have only found fares (for 2 pax) on 1 date during the period, either all sold out or very limited release of dates. If you look at the sort of free publicity these sales buy - release say 50 seats at ridiculous price and get free PR on national news - then it's a good reason. Consumer looks, and either has one of two responses, anger they can't find such fares, or realisation that they can get $49 o.w. fares on same sector which isn't bad so buy them instead. Very clever marketing.