They probably have logic built into the system that accounts for supply and demand, season, special discounts, etc.
For example, there might be a trigger that fires every time a ticket is bought for a flight. When there are are 10 seats left, the trigger might increase the price by 20%. Another process might constantly search for available flights on different airlines on the same schedule and destination, and adjust their prices accordingly. Another process might check to see if there are 8 or more seats left on a plane that leaves within three hours, and discount 30% off the pirce of those seats, just to get them filled (empty seats make no money for the company, and the cost of the flight is still the same..) You get the idea.