Just a simple "teaching" post.
There are two different ways of thinking about central bank profits (of doing the accounting). A simple numerical example will illustrate the difference.
Assume 2% inflation, and 1% real GDP growth, so Nominal GDP grows at 3%. Assume the stock of currency is 5% of annual NGDP. Assume the nominal interest rate on bonds is 4%, but currency pays 0% interest. Assume central banks have zero operating costs.
Method 1: Every year the central bank prints currency worth 0.15% of NGDP (that's 5% x 3%) so its profits are 0.15% of NGDP, and it gives those profits to the government which owns the central bank.
Method 2: When the central bank prints currency it buys bonds, so owns bonds worth 5% of NGDP, and earns interest on those bonds equal to 0.2% of NGDP (that's 5% x 4%), and it gives those profits to the government which owns the central bank.