Orders reports are structured like other SellerCloud reports, but differ in that they include unshipped and shipped orders, and use the Order Date for date selection results.
In contrast, Sales reports are akin to Profit reports and deal only with shipped orders, using the Ship Date as the primary date selector. (Unshipped orders may not be filled, which could result in cancellation and thus, no profit.)
Example: If you receive orders over the weekend, your Sales report will not show how many orders you received on Sunday, a non-shipping day. Rather, it will show them on the day they ship. By comparison, your Orders report will show orders on the the day they were ordered (Order Date).