JMJ,
Despite what people think, the actual "full fare" prices don't fluctuate very often at all.
Lets say for arguments sake that the full PH-SY return fare was $1200.00 (not far from the truth). Well the airlines obviously are not going to sell all of the seats on the aircraft at that price, so they offer fares with restrictions at lower prices, e.g. you can often get a 21 day advanced purchase fare for 50% of the full fare - when added to by taxes etc, that comes out at around $650.00, and the same goes for 14 day and 7 day advance purchase fares, although they increase to maybe 60% or 75% or maybe more. But they are all based on the full fare.
However, even if you call 2 months ahead, the airline might not be able to give you that cheap 50% fare, because they have already sold the number of predetermined seats at that price.
You can also often find that on the day of departure the fares come back down toward the 50% level, so that the airline can sell those seats off rather than have them empty.
I hope that my fairly long winded answer makes sense.
Now to put it all into perspective in the current local aviation market, that is why you have had people whinging about QF putting fares up because AN collapsed. Not technically true, they have simply sold all of the preallocated cheaper seats, and the next available fare is what they have been quoted.
Take It Easy,
OneDotLow