U.S. taxpayers are in line for a big payday, but it is not quite what it seems.

The Federal Reserve is out with its year-end tally of net income Tuesday, and the central bank says it expects to return $76.9 billion to the U.S. Treasury. While that may seem like a windfall for Uncle Sam, a large portion of the figure amounts to taking money out of one pocket and putting it in another: interest payments on U.S. government bonds account for a sizable chunk of the income.