Alaska Air Group carried 271,000 more passengers in March 2015 than in March last year

Alaska Air Group’s two airlines saw double-digit traffic growth in March, the SeaTac-based airline holding company announced Thursday.

That 11.2 percent growth in revenue passenger miles, however, slightly lagged the air group’s 12.4 percent capacity bump in March. Alaska Air Group owns Alaska Airlines and Horizon Air.

That mismatch between capacity and passenger growth resulted in Alaska’s planes flying with a greater percentage of empty seats last month, 13.9 percent compared with 12.9 percent in March 2014.

The company’s on-time performance fell in March to 85.6 percent compared with 88.2 percent a year earlier. On-time arrivals and departures are those that arrive or depart within 15 minutes of their scheduled time.

Alaska has been aggressively expanding its route network in part to counter the efforts of Delta Air Lines to serve more cities from Sea-Tac Airport, Alaska’s hub. Delta is feeding a bevy on new international flights connecting Sea-Tac with destinations in Asia and Europe.