The main point of each iteration is to complete stories. If the customer brings ten stories, and the programmers brainstorm ten tasks for each story, what should they do if they're only able to complete 90 of those 100 tasks? One possible thing would be to get each of the ten stories 90 percent done. We think that's not going to impress the customer. In fact, we hereby instruct the customer: accept only real completion on your stories. Ten stories 90 percent done equals zero stories done.
A better way would be to get nine of the stories completely done and skip one—the least valuable one. Nine stories 100 percent done plus one story zero percent done equals nine stories done. Much better than zero.