Items listed on booksamillion.com as "On Order. Usually ships in 2-4 weeks" are Backordered items. These items must be obtained from the publisher so the warehouses can fulfill orders placed.
Backordered Items: Cannot be fulfilled at the current time due to a confirmed lack of stock at the warehouse. Generally, once an item is placed in backordered status the warehouse notifies the publisher who is then required to send more or confirm the item is out of print. Sometimes an item's status on bam.com may state "In Stock" but the item later is placed on backorder. This is due to the website and warehouse system not being up to date with one another or the warehouse's system not accurately reflecting the current quantity.