Bigbasket delivery guys asking to return missing items

I had stopped ordering from Bigbasket because of this. After two years, I made a few orders and again ran in to this. When I place the order, the item is in stock. When the delivery comes, some items would be missing. The delivery person tells me that they were out of stock and I need to raise a return request for the missing items and it will be accepted and I will get the refund. When I ask them how can I return the items that I haven't received, they would simply say, no return needed. When I ask them why don't they cancel the item while packing if they are out of stock, they would say, I don't know sir, the guys at store said so. So I usually raise a problem with the order, tell the customer support about this. They would refund the money, but are silent on why it is done this way. Initially I used to wonder if the delivery guys are stealing the items. That might not be the case. I am curious though.