The average is called a rolling average because after the shipper reaches the specified period, i.e., 52 weeks–their charges from older agreements are dropped off, and those for newer ones are added in. So, for instance, an invoice received on the 53rd week of one such agreement would use totals calculated thus far (1-52 divided by fifty-two) as well as any new data that has come into play since then; this gives you more up to date information than simply looking at last year’s figures alone!