Interest owed on 2018 tax refund over-payment

I received a federal tax refund for my 2018 tax filing ($3,400), but later discovered an error ($747) that resulted in a reduction of the refund (corrected to $2,653).   The amended return and re-payment of the $747 was made in May of 2021.

I just received (4/7/2022) a notice that I owe the IRS $71.86 interest --- presumably on the $747.  Would appreciate clarification on the interest being charged and how it is being calculated.  I discovered educational account tax forms after the fact which revealed the addition income and wanted to make the correction as soon as possible.  I gather that the interest owed is the "reward" for doing the right thing.  🙂