It is easy to forecast how many widgets you can ship when you are creating them in the factory behind your office.  It is another story when those widgets are being created in another factory that you do not own.  You are now a customer of your distributor, but as a market economy would dictate, your customer's do not care where the product comes from.  They just want to know if you have it in stock, and if you do not, they would like to know how soon you will have it.  Any company that ships a product that they do not produce themselves runs into this problem consistently, and often enough you will need to connect you systems to your distributors to get a virtual inventory to keep your customer happy.