Before Delta (DAL) - Get Delta Air Lines, Inc. Report filed for bankruptcy protection in 2005, its revenue per available seat mile was 86% of the industry average. After it emerged in 2007, Delta began to see a revenue premium -- in other words, its RASM was above 100% of industry average.

This has continued to be a source of pride for the airline, which sees its 109% first-quarter 2017 revenue premium as an indication of its superior appeal to passengers.

But United (UAL) - Get United Airlines Holdings, Inc. Report President Scott Kirby questioned the validity of the revenue premium metric.

"I hate that metric," Kirby said last week during United's first-quarter earnings call, after being asked whether United also has a revenue premium.

"If we calculate the numbers here at United, yes, we have a revenue premium and yes, it's higher even than Delta," Kirby said. "American also, by the way, when I got there said they had a revenue premium.

"Everyone makes all these adjustments to the numbers on stage length and density and a whole bunch of other stuff and so with all the caveats everyone calculates that they have a revenue premium," Kirby said.

"You can make your revenue premium go up by taking a row of seats off airplanes, for example, and that makes your RASM get higher," he said. "You could fly only first-class seats on airplanes, and that will make your RASM higher and will give you a revenue premium and have a real cost problem."

It is more worthwhile to look at margin, Kirby said: "We are focused on margin, in closing the margin gap and improving our absolute and relative margin."

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Delta calculates margin premium by comparing its RASM to the industry average compiled by the Airlines For America trade group, said spokesman Morgan Durrant.

On Delta's first-quarter call, an analyst asked CEO Ed Bastian whether the revenue premium is sustainable.

"We have been averaging in the 106 to 110 range of industry RASM for the last number of years," Bastian said. "I don't believe it's a temporary phenomenon.

"We have heard this probably for about eight years or nine years, that Delta's revenue premium was somewhat at risk and every year we continue to go to market and prove that the Delta people are the best in the business and our customers value and pay us a premium for the services we provide," he said.

Aviation consultant Robert Mann said revenue premium to other airlines isn't necessarily meaningful because it is impossible to accurately reflect what one airline would produce while flying another airline's network.

However, he said, for Delta, "The change from {pre-bankruptcy} is meaningful. Going from a deficit to a premium says a lot."

For Delta, it is all the more significant, Mann said, because it has occurred as Delta has been flying bigger aircraft, thereby spreading its revenue per available seat mile over more seat miles.

"They get a unit revenue premium even with a larger gauge network," he said.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.