If you defined a percent increase over the Unit Quantity (to account for loss or damage to the order quantity) in Ship Overage Definition, then the Shipped Quantity field on the Buy Module would show a larger, calculated amount than the Unit Quantity field, when this insertion order is trafficked.
Once additional materials are trafficked, the new values or 0 replace the previous Shipped Quantity before the newest trafficking.
The increase may be viewed on any of these traffic Shipping instructions: PDF or exportable.