Why are the suggested quantities individual items on a forecast group purchase order different from the items on the order plan?

Generally speaking, the total forecasted demand for all items in a forecast group should match the sum of every individual item (the lead demand, or forecasted demand, plus each total or forecast per month).

However this may not be the case is the forecast group has been configured differently than the individual items on the order plan (eg: with a different lead time, service level, growth amount etc). 

Also note that the recommended order quantities may be different - this will happen if you have several items in a forecast group that have stock on hand, but have never been sold: the system will calculate a 0 order quantity at the individual sku level on the order plan, but these will be added to the suggested order quantity when creating a purchase order from a forecast group. So in this scenario, the suggested order quantities of each item in the forecast group will be adjusted so that they add up to the total recommended of the forecast group, including the suggested orders of the new never-sold items. 

In either scenario, it is at your discretion, about what is best for your business whether you place orders calculated individually or via the forecast group. If your demand usually is in product ranges, and your customers pay less attention to the individual SKU's then we'd recommend following the forecast group based suggestions. Otherwise, if your customers pay attention to the individual SKU items then follow the individual suggestions at the SKU level on the order plan