Job Posting Group on the Job Ledger Entry table

Hi A client posed a question that I have been unable to answer so I am appealing for your assistance. The client is working on GB v2.5. When an inventory item is purchased with a Job specified on the purchase line then the Job Ledger Entry field Job Posting Group ends up with the Posting group of the Inventory item. My question is why? There is no inventory ledger posting in these circumstances and the Job Ledger Entry Gen. Prod. Posting Group has the correct value from the item. Hope you can help.