The federal government reported Friday that on average, its employees are underpaid by 26.3 percent compared with similar non-federal jobs, a "pay gap" that increased by about 2 percentage points over last year while federal salary rates were frozen.... The pay gap in the Washington-Baltimore area was calculated at 36.9 percent, slightly below the 38.1 percent reported last year. Officials said a variety of factors could have caused that result, including changes in the mix of jobs and switching thousands of Defense Department employees into the general schedule and out of a separate pay system that is being phased out....