caphis

Although a flight may "cruise at 36,000 feet," it typically will not take off and immediately proceed to that altitude. A JFK-LAX flight, for example, may file for a cruise at 36,000 feet, but may take off from JFK and top out at 32,000 feet until, say, Kentucky. From there, it may climb to 34,000 feet until, for example, Oklahoma. At that point, enough fuel has been burned to make cruising at 36,000 feet efficient. If a fully loaded A320 took off from JFK and proceeded directly to 36,000 feet, it would not be operating as efficiently as possible because of its initial weight. The actual flight plan and climb profile will vary depending on the weather factors involved.

A fully loaded 737-800, by contrast, is able to climb to those more efficient fuel burning higher altitudes quicker than a fully loaded A320.

I hope that's helpful... I'm clearly not the best person to explain something in a non-confusing way. ;-)

ETA: As an example, check out the profile for JBU671 JFK-LAX on February 10. As you can see, the flight climbed to 32,000 feet and remained there for ~40 minutes. From there, it climbed to 34,000 feet for another 90 minutes. After that, it climbed to 36,000 feet for the remainder of the flight. GloberTripper means that the 737-800 is, generally, able to get to that 36,000 cruise quicker than the A320, and thus operate a bit more efficiently.