Different airlines are competing with each other. They all work in the same market so as with most markets, they range from economy to expensive. If you pay more you'll get a larger plane, which means more legroom, better meals, movies and maybe even a better pilot.
There are many other factors that will change the price from day to day, even hour to hour. The closer to the flight your booking time is the cheaper it will be because the airlines want to fill the plane up. One airline may be near capacity, while another has barely sold a ticket. Of course the airline with more seats available will charge less for their flight, they want your business more than the competitor.
You may even notice that agents from the same agency will differ on price. There are even many answers to explain this phenomenon. Agents will go out and look for deals for you from the various airlines. They have many travel arrangements to look for so they may not dedicate enough time to finding the cheapest one, while another agent may already know of a deal related to what you're looking for.
Fuel will always be a factor as well. The rise and fall price of gasoline will cause prices to fluctuate at unpredictable rates.
The best you can do is spend the time yourself looking for the best deal, once you have a good comparison, pick one. Or you can always wait till the last minute and book something for really cheap. (www.lastminuteclub.com)
