At first (and second and third read throughs), that was clear as mud. Then I read what you wrote on the link and I think I get it now.

Defaulting is failing to submit the story, like defaulting on a credit card. This date is designed to give you time to find a pinch hitter.

Works due is the actual due date for the story, and at that time, we click the "fulfill" button to submit the completed story.

Is that right? :) Sorry to be so dense! :)