Discount on receipt for AR Invoice not calculated correctly

I have created a AR invoice manually with amount 1200 USD and due date of 30 days. However there is a discount of 3 percent, if the amount is paid within 10 days and discount is also applicable for partial payment as well. Therefore I manually created a receipt of 400 USD and applied to the same transaction. I expected a discount to be 12 USD but the system has calculated as 12.37 and overall balance is 787.63. Why is the discount calculated as 12.37 instead of 12.