If you're doing it on another day, the inventory (and thus price) probably changed. The price of a seat on a plane is dependent on how many other seats have been sold.
Sorry, my wording was confusing. I'm saying that on another day if you repeat your search you might get a higher price. Then, clear your cookies and search again- boom, back to the lower price. I've seen it happen 3 or 4 times now.