Base Currency / Default Display Currency / Allowed Currencies are all US Dollar. This was happening with random promocode orders, then today I see one with no promocode. Everything I research points to a currency calculation, yet we are using only US Dollars. Today, the attached examples are whole dollar differences, but previous orders have had discrepancies of a few pennies - some with a higher base_grand_total but most with a lower value.
Click thumbnail to see full-size image