It's an accounting thing [1]. When the Fed makes a profit, it remits it to the Treasury [2]. (I believe this is an anachronism from the gold standard days, but not sure.)
[1] https://www.stlouisfed.org/on-the-economy/2018/september/fed...
[2] https://www.wsj.com/articles/fed-sent-88-5-billion-in-profit...