User has set the rates and units in Core to display a certain number of digits after the decimal point in Global Settings > User Interface screen, but it is not obeyed for Cost Rate in Expense Entries screen. It always shows four digits after the decimal point.
This is the expected behavior in Core. While the Global Settings apply to most of the rate fields in Core, the Cost Rate field in Expense Entries is an exception and it is configured to always use four decimal places. This is to accommodate the expense items that may have granular calculations, e.g., mileage. Check the Core Help for more details.