Fares change every day. Price is a function of capacity, actual load, projected load, and a number of other factors that change multiple times a day. Based on this, the prices you see change. Seats on an airplane are just like any other futures market - sometimes you get a good deal, sometimes you don't, but whatever you locked in at is what you locked in at.
The only thing that I can think of that you might be thinking of that may have confused you is the NWA.com best fare guarantee, which says that if you find a better fare on the same flight and date they will give you a credit - used to be difference + $50, now it's difference + $100.
http://www.nwa.com/features/bestfare2.html