Vendors may have a minimum purchase amount per product, i.e. product x has a minimum qty of 6 that must be ordered.
On a product toolbox > Purchasing page, set the minimum qty for PO. This setting will prevent the item from being added to a PO with less than the minimum qty.
In addition, the predictive purchasing calculations will factor in the minimum qty when returning the suggested order amount. For example, an item with a minimum PO qty of 6, that has a real suggesting of 15, will return an order qty of 18 (as a multiple of 6).