Expanded distro sales will tend to show up in spurts because they are not live-time sales to customers. They are sales to retailers. If BN orders three copies to keep in inventory, they will show up all at once. It might take them three months to sell those copies to customers, but the sales have already been posted to you. Also, sales to actual brick and mortar stores are usually reported monthly through Ingrams, so if Jane Doe Bookstore bought four copies on December 1 and Jake's Bookstore bought 1 copy December 10th and Alice's bookstore bought 1 copy December 24, all 6 might show up at once when Ingrams files their report January 10th.