When a supplier receives an order, they will often order excess raw material for production. This is because some product will be lost over the course of production, often due to defects, and this helps make sure they hit the target quantity of the order. This results in the final quantity being either over or under the target quantity outlined in the PO.
When placing an order, we make buyers and suppliers agree to accept a minimum over or under of the target quantity by default. This is standard practice for the industry.