Does a bill created with no PO and IR updates Inventory?

We just noticed that once a PO is created and recieved, and at the time of billing if we add a different item which is not in the original PO or IR it updates inventory of that item.

We also noticed that inventory is not updated if simply the quantity is changed which is not there in PO
Example: Item A qty 2 ordered-Received only one in NS and billed for 2. Inventory is updated as only 1.
Item A qty 1 ordered-Received 1 and billed for 2 (if we receive in actual 2). Inventory is updated as only 1.
but
Item A qty 1 ordered. Recieved in actual Item A qty 1 and Item B qty 1. Recieved in NS Item A -qty 1. In the bill added the Item B. It updates inventory for both Item A and Item B.