Federal employee salaries on average are nearly 27 percent below those of their private-sector counterparts, a difference that narrowed over the past year for no readily apparent reason, an advisory council reported Tuesday.

The “pay gap” number, released at a meeting of the Federal Salary Council, is based on Labor Department data on salaries in a range of occupations, levels and local areas. Federal employee unions and other organizations commonly cite the number in seeking substantial federal pay raises, though in practice, the annual increases have been much smaller.

The report shows that as of March, the average difference was 26.71 percent, down by 4.25 percentage points from the year before even though federal employees received raises averaging only 1.9 percent this year.

Council Chairman Ron Sanders said he would ask the Labor Department for an explanation. “Some of these are just statistical anomalies,” he said in an interview after the meeting. “How can we tell whether one, especially one that is significant, is real? We don’t have the answer to that. But that’s an answer that I want.”

For example, in the Laredo, Tex., area, the indicated gap increased by more than a quarter, which the report suggested was skewed by changes in salaries in law enforcement, a major occupational group for federal employees there.

Sanders is a longtime senior federal personnel official who is now director of the School of Public Affairs at the University of South Florida. The council also includes several members representing the administration and others representing federal employee unions.

Under federal pay law, the figures are to be used to bring salaries up to private-sector levels as measured by a formula in that law. Those amounts vary in turn among some four dozen metropolitan zones, with a catchall “locality” for areas outside those zones. However, disagreements over the calculations and the potential cost have prevented the indicated raises from being paid.

Other studies, using different methods, produce different conclusions. Some conservative and libertarian organizations have found a difference in favor of federal employees, while the Congressional Budget Office in 2017 found salaries overall to be about comparable, although with variations by educational level.

In an August letter to Congress backing a 2.6 percent federal employee raise for January 2020, President Trump said that following the law would cost $24 billion “in the first year alone.”

A union representative said that whatever the exact size of the pay gap, its impact could be seen in testimony at the meeting by both agency and union officials from a number of places in the catchall locality asking for higher pay rates. They cited difficulties in recruiting and retaining employees in law enforcement, medical, scientific and other occupations despite using shortcut hiring rules and financial incentives.

“We’re really feeling the impact of three years of pay freezes and the minuscule pay raises since,” Jacqueline Simon, public policy director of the American Federation of Government Employees said in a telephone interview. “It’s time for a substantial makeup. Since the Congress and President Trump have refused to do that, you still see this substantial lag.”

Federal salary rates were frozen over 2011-2013, and annual raises afterward have been in the 1 to 2 percent range.

The federal raise for 2020 remains undecided. The Senate has drafted a spending bill that effectively endorses the 2.6 percent figure, which would be paid with no variation by locality. The House has passed a bill to provide a 2.6 percent increase across the board plus an average of 0.5 percentage points for locality-based pay. That would result in raises varying from somewhat below to somewhat above 3.1 percent, based on figures in the Salary Council report.

The San Francisco area would stand to receive the largest increase even though it already is the highest-paid locality, followed by the Laredo, Los Angeles, New York and Washington-Baltimore areas.