Order fill rate counts an order as filled only when every line on it shipped in full. It is what the customer experienced, since an incomplete order is an incomplete delivery whatever the unit fill says. It sits well below case fill on any business with multi-line orders, and the gap between the two grows with lines per order.
Orders on which every line shipped complete, divided by orders shipped.
Of four orders, two shipped every line in full: 50 percent. Unit fill on the same orders was 89 percent.
Unit fill reported to a customer who experiences order fill.