in my case we have order related billing, in which case we don't have delivery.
in order we have item numbers as "1, 2, 3.""
but, when it is copied to billing, the item nos are getting as 2,3,4"
and the item '1 'is copied as item '4.'
I am not able to find, why this strange behaviour.
I've maintained the copy controls at item copying requirements from order to billing as '001' at header and '002' at item level.