NEW YORK (Reuters) - American Airlines Group Inc AAL.O on Thursday became the second major U.S. airline this week to lower guidance on a closely watched revenue metric, sending shares of several major carriers lower.

American Airlines aircraft are parked at Ronald Reagan Washington National Airport in Washington, U.S., August 8, 2016. REUTERS/Joshua Roberts

However, analysts said the forecasts for lower unit revenue at American and Delta Air Lines Inc DAL.N had different causes, and were not a sign that the air travel market is hitting an unforeseen slump.

American shares were down 3.48 percent at $43.33 at market close, while Delta and Southwest Airlines Co LUV.N fell less than 1 percent and United Continental Holdings Inc UAL.N closed down 1.2 percent on a day when U.S. stocks gained slightly.

“Things are improving, but at a slower rate than airlines had hoped for,” CFRA Research analyst Jim Corridore said, blaming recent market dips on the slow trickle of reports on carriers’ performance.

American said on Thursday morning before market opening its unit revenue would rise 1.5 percent to 3.5 percent, cutting an earlier projection that total revenue per available seat mile would rise 2.5 percent to 4.5 percent.

“It’s still trending positive. (Revenue per available seat mile) is still rising, and that’s the break from the negative trends we need to see,” Corridore said.

Share prices sectorwide also slipped on Monday following Delta’s trimmed outlook.

American said its unit revenue fell mainly because it canceled fewer flights last month than it did the year before.

The company said it completed 98.9 percent of its mainline flights last month, compared with 97.7 percent in the same period last year.

American’s improved completion factor, the percentage of an airline’s flights that were completed without cancellation, while good for customers, ultimately put a drag on the company’s expected unit revenue rise, American said.

Delta, for its lowered guidance, cited a “more moderate pace of improvement in February.”

U.S. carriers have struggled to mark positive increases in unit revenue, which measures sales relative to flight capacity, as cheaper fares and tougher competition have pummeled the industry. A positive increase in the metric would break a two-year streak of flat to negative change. In the same statement, American reported February 2017 total revenue passenger miles fell 3.3 percent from a year earlier to 15.2 billion. The airline said its total capacity fell 3.7 percent to 19.6 billion available seat miles in the same period.