"On Order" means that we don't have the particular title in stock and order it from the publisher as needed. The time it takes to stock such a title is not in our hands and depends on the speed of the publisher.
We have set an internal Average Delivery Time for every item. If this is exceeded, a title gets listed as "Delayed". If that ADT is exceeded by a larger amount of time, the title gets listed as "Delivery problem".
-Peter

