Have you ever gone to a restaurant and been told there will be a 30 minute wait? If the hostess seats you after only waiting 25 minutes, you are usually happy. You were able to get seated 5 minutes faster, than you originally anticipated.
Now what if the original wait time you were told was 15 minutes and it still took 25 minutes for you to be seated? You are no longer happy. In both instances it took 25 minutes to be seated, however your initial time or (anchor bias) differed therefore producing different reactions.