First the item API does not return information about all items. For example:
http://us.battle.net/api/wow/item/38682 (this is Enchanting Vellum)
http://us.battle.net/api/wow/item/39501 (this is Heavy Parchment)
http://us.battle.net/api/wow/item/39502 (this is Resilient Parchment)
All the above item API request result in 404 HTTP response with the following result:
{"status":"nok", "reason": "unable to get item information."}
However those items still exist in game and some of my characters have these in their inventories.