"Order Items"."ID" AS "OrderItemID", ... (("Orders"."ID" || '-'::text) || "Order Items"."ID") AS "FileKey"
Wait, then why are the Item IDs different?
Here are the results again, with the FileKey field expanded a bit.
I don't understand what you are questioning...FileKey is a hyphenated concatenation of the ORDER Id and the ORDER ITEM Id, while OrderItemID is just the later - and the suffix of the former matches the later.