There is a very basic reason for the reduce of fuel consumption at higher altitudes in jet engines. The easiest way to put it is - the air is less dense at higher altitudes. I could leave the answer at that, but I think for sake of clarity I will elaborate some. The most fuel is burned during takeoff and climb. The reason, as I mentioned, is that the air is more dense in lower altitudes. The second reason is that you are overcoming drag and weight during takeoff. If you think about a semi truck taking off from a red light, one will notice how much harder the engine must work to get the load moving. It requires a geared down engine with a lot of torque. If we compare this to a commercial jet, say a 757, it's not too much different. This jet could potentially weigh 250,000 lbs. (depending on configuration and load) It takes a lot of fuel and power to get that much weight moving at a high enough airspeed to lift off. Furthermore, once off the ground, the aircraft will be travelling at approximately 160-200 knots initially. It will gradually increase to 250 knots as it approaches 10,000 feet. Now, lets analyze a few points about this takeoff and climb. Obviously, we know that the amount of oxygen on the earth's surface is considerably higher than say, 30,000 ft above sea level. This makes the air thicker and more dense, because the concentration of gases is higher. Consequently, the amount of drag on the aircraft is considerably higher because there is more air to push against. Also factor in that all aircraft obey a 250 knot speed limit below 10,000 feet. Slower speed and more drag equals a higher fuel consumption. Once at cruise altitude, things change. Most large airliners cruise anywhere from 32,000-42,000. Smaller jets, such as the CRJ, are capable of these altitudes, but due to shortness of route, often say in the 20,000-32,000 range. By the time the jet reaches this point, it could be travelling at 430+ knots. Combine this with thinner air and you equal a need for less thrust to stay airborne. You have the speed already going and you are fighting less resistance. This is the point when the jet engine is most economical. Cruise power will be set and autoflight fully functional. Lastly, with the increase in speed, comes a greater distance travelled per pound of fuel burned. In conclusion, the difference in air density combined with greater speed cause the jet engine to be more efficient at higher altitude than at lower altitude.

the tail wind helps carry the aircraft so less fuel is used. all that data and no points for you, shame on that fly guy.