Why did my federal amount due go down when I entered my stimulus information if the stimulus isn't supposed to affect your refund?

Everywhere I look it says it won't affect your refund or increase what you owe.  But when I entered that I got my two stimulus payments the 'Federal Tax Due' shot down and I'm left with a potential credit to be approved by the IRS.  In my mind any credit should be aside from what I would be getting outside of filing any stimulus information.  Can someone explain why if I had not entered stimulus information I would have received a different refund if it's not supposed to affect the refund?