Sometimes, it's not obvious why a particular item (or the quantity of that item) has appeared on your purchase order e.g. if you've created a purchase order based on reorder levels and restock points.
In the purchase order grid, there's a little (?) button designed to help you with this:

If you click this (?) button, a popup will appear, detailing how the quantity was calculated:

In this instance, only one unit was added to the purchase order, but the manufacturer supplies these energy chews in boxes of 18, so an extra 17 units were added.