Don't get all your stories done?

Read something interesting today in Scaling Software Agility

But at the high end, which looks like excellence incarnate, additional tuning may be required, because if a team truly hits 100 percent of stories on every iteration, then it is likely taking on too little work or deferring technical risk-laden stories until a later time. Effective agile teams typically run between 60 to 90 percent story acceptance, and even that percentage will vary significantly from iteration to iteration.

I have never read this before. I always thought you were supposed to get all of your stories done. He makes a good point that we are probably deferring technical debt to a later time. I thought you were supposed to do this though. I will keep looking around and see if other people agree with his statement.