I just got back from WDW and talked with a friend who ate at Be Our Guests a few days ago. She was unable to get the ADR, no matter how many times she checked.
She decided to try for a walk-up by going just before the restaurant opened for dinner if they were accepting any walk up guests for seating. The cast member claimed they don't take walk ups, but my friend could wait for a "cancellation".
And within 15 minutes, she was seated. So, we believe Disney cast members are leading guests to believe that miraculous walk up seatings can happen if a "cancellation" opens. We have a theory (pure speculation on our part) that Disney is booking ADRs for a percentage of open seats and then fitting in walk ups so there are no overloads of guests.
We could be wrong, but that's the way it seems. My friend suggested trying to arrive before dinner seating, and the cast member may or may not seat you, but it never hurts to ask.