If your salary is $5k a month, and you get an advance of $1k, then next month you'll be paid only $4k. You have a fixed amount of money that you're owed. Any money you get now is money you're not getting later. Similarly, you were owed $2400 of tax credit, but you already took that money, so that amount is subtracted from the amount that you're given now (or added to the amount you have to pay). It's not that the tax credit increases tax owed, it's that the fact that you already took the credit increases your tax owed, compared to if you still had the tax credit available to apply towards your tax owed.