Supply and demand. The airlines have a crazy formula for airfares and use this formula to capitalize on the maximum potential revenue they can earn. Airlines start with pricing and set all the fare levels and put them in separate fare classes that are hidden from customers. The airlines then decide how many seats to sell in each class on every flight. Depending on a lot of factors, the airline’s system sets a limit and then sees that a flight is booking faster than predicted, so it may clamp down on the number of cheap seats out there because demand is higher than expected. On the other hand, if it’s really slow to sell, the system may open up more cheap seats to stimulate demand.