Sorry it makes perfect sense. You had a 600 credit on your return. If you take it off you don't get it and your return should have been $600 more due.
We don't know why you ended up owing. But usually your withholding wasn't enough to cover the tax on your income. Look at your return. What is your income on line 15? What is the tax on line 16? Then subtract your federal withholding. That is the amount you still owe or get back unless you have other credits like the EIC or Child Tax Credit.
