Why do some orders have line items split out into separate fulfillment items and not others

[Brian_S]

I'm trying to understand the business logic behind Shopify's splitting of line items into separate fulfillment items. 

I have an app that will add new line items to an order after the order has been created.  Sometimes the line items are added to the original fulfillment item but sometimes they're added into a separate fulfillment item. 

All the fulfillment and shipping rules are the same. This happens even when the added line items are duplicates of the items already in the order. 

It seems like it happens when there's an item with a selling plan (ie subscription item) more often but I think I've seen it for items without selling plans. 

Any ideas or links to documentation around this logic would be greatly appreciated!