1. Internet travel sites and travel agents use the central reservation systems which are updated periodically during the day. The four are not updated at the same time, which explains why different searches may yield different results.

2. The systems also may use different algorithms to search for the lowest fares, which subsequently provide varied fares.

3. Demand explains why it is cheaper to fly on a weekday (when fewer people are traveling), at odd hours, or on days other than major holidays.

4. Airlines change their prices based on competition. If one airline flying the New York-Miami route drops its rates by 20%, chances are all airlines will drop their rates, so as not to give the discounter a competitive advantage.

5. If one airline has the market share for a particular route, it may not need to lower fares to attract passengers. Other airlines with less market share will be willing to offer lower prices to attract customers.

6. Prices change incredibly quickly. Prices for a specific flight can go up or down even as your travel agent is getting your credit card information from you, so your cheap ticket can be whisked out from under you. And your fare is never guaranteed until you have paid. That's the gamble. Fortunately, it can work both for you and against you.

7. Airlines also use inventory to their advantage and to lure the unsuspecting flyer. They divide seats on each flight into several price ranges and set aside a certain number of discounted tickets. Naturally, the lowest fares draw your attention to advertisements. Of course by the time you call (unless you are quick on the draw) those seats will be gone. Inventory brings us back to demand. If there is low availability and high demand, you will have to wait for a cheap fare. But that does not mean you should give up on a trip that is very popular. Sometimes airlines will change their fares or open up more discounted seats, depending on how sales are going.

SoYouWanna know more? Check out our full-length article SYW backpack through Europe?