I don't know for sure, but I think WN has a lot of their customers money upfront, since they don't have a change fee on any reservations. So, If I don't know which day I would fly for sure, I buy flights on 3 days in a row in advance and then just reuse the money from 2 flights that I didn't use in the future. So, since I'm "hedging" my flights, they use my money to hedge fuel.
