Inventory dimensions on BOM and Production order


When I create a new Production order I indicate the required Inventory dimensions (Site, Warehouse, Location, Batch number). I also created the BOM for the item I want to produce. This BOM has only one line and no Inventory dimensions are specified. Nevertheless, when I click the BOM button on the Production order form, the BOM line shows the Inventory dimensions that were specified for the production order. These are the same dimensions that are registered on the Picking list journal. Why and how does this happen?