When rounding numbers, Checkout uses a rounding method called Banker's Rounding. Banker's rounding rounds .5 up sometimes and down sometimes. The convention is to round to the nearest even number, so that both 1.5 and 2.5 round to 2, and 3.5 and 4.5 both round to 4. Values other than .5 are rounded using the standard rounding technique.
Internally, Checkout stores numbers with up to 16 decimal places. Checkout displays numbers with just the amount of decimal places that can be expressed in the current currency, and the banker's rounding method is applied to generate that display. When you export your financial transactions from Checkout to a Numbers, Excel, comma-separated or tab-delimited file, the actual unrounded data is exported. In Numbers and Excel files, Checkout applies a number formatter to the fields containing monetary amounts, so they display as two decimal amounts in the spreadsheet, while leaving the precision of the data intact.