LATEST_SHIP_DATE and BILL_TO_ID on Purchase Order

Can someone clarify two fields on Purchase Orders/Shipments:

1)

LATEST_SHIP_DATE - according to data model: The date after which the item can not be shipped by the supplier. NOT_AFTER_DATE - date after which the shippment will not be received. And I found that LATEST_SHIP_DATE will be calculated as NOT_AFTER_DATE+SYSTEM_OPTIONS.LATEST_SHIP_DAYS.

The confusing thing is: LATEST_SHIP_DATE will always be higher than NOT_AFTER_DATE. This can mean that, for example, we won't accept shipment after 15.10.2015 (NOT_AFTER_DATE), but supplier might ship it by 20.10.2015 (LATEST_SHIP_DATE).