From my experience, the difference between "unfilled purchase orders" and "order backlog" is

Unfilled purchase orders = customer orders

Order backlog = customer orders + marketing orders

Marketing can place orders on the factory in anticipation of customer orders. Marketing orders are often done when factory capacity is near it's limits and marketing has some customers who are very serious but haven't signed purchase orders yet. These customers are expected to want the product very quickly aftrer they sign so marketing places an order on the factory allowing them to buy materials and schedule production.