It's a fact of life with any airline that prices fluctuate according to supply and demand, season, and whether there's a seat sale on. If you want a cheap deal, have a surf around as far ahead as possible, to get a feel for what's a good deal, and when you spot something that looks good, get in there quick!
The less time you spend doing this, the less opportunity you have of getting a good deal.
I booked a flight from London to Auckland last week for £664 return. £664 return to the other side of the globe is good fare by anyone's standards, but a friend of mine booked today with another (better) airline for £630. That fare wasn't available at the time I booked, and I'm sure that there will have been people booking the same flight as him for the £800+ fare they were quoting last week! That's their hard cheddar cheese, just the same as it'll be my hard cheddar cheese if I end up sitting next to someone who's got a better deal than me for my flight.
That's just life, I'm afraid, and there's absolutely no point in getting bitter and twisted about it. Next time, just try and make sure it's you who pays the cheaper fare!