Every year, tens of thousands of federal employees retire or otherwise leave their jobs. But some agencies have much higher turnover rates than others. That data nugget is buried in a recent Government Accountability Office report examining government workforce trends. From fiscal 2004 through 2012, the average annual government retirement rate was 3.5 percent, the average resignation rate, 2.4 percent, for a combined “separation rate” of 5.9 percent, according to the report. But when GAO reviewers looked at 24 individual agencies, they found a pretty big spread around that average. During that 2004-12 period, for example, the Environmental Protection Agency…