If a student hasn't submitted a test, the scores page shows a zero. But in fact the actual score, as shown in Gradebook, is not submitted. Conceptually there is a difference between unsubmitted and submitted with 0 grade. We've had teachers want to assign an actual 0 score for someone who hasn't submitted. There's no way to do that. You'd normally go to the scores page and set the score to zero. But the score is already shown as zero there. So the only way to create a real zero score is to set it to something non-zero and then set it back to zero.
I believe the right approach would be to change the scores page to show either - or blank for unsubmitted assignments. Then if a faculty member enters 0 and hits Update it would be a change.