Closing/Open inventory question

Hi Sorry if this is a stupid question but I am confused. When calculating cost of sales in marginal costing.. the proforma I have learnt is variable cost of sales= +variable production costs +opening stock - closing stock etc etc.. However when working on the topic budgeting. To work out a production budget the proforma is Sales +closing inventory -opening inventory = production quantity. I know both examples are different and calculating two seperate things but Why is in that in the MC proforma when calculating COS we -closing inventory and + open inventory, but in the budgeting topic when calculating production budget we +closing inventory and - open inventory? Can you help me by explaining the logic behind both. This might be going back to basics but I am confused and it in causing confusion. Thanks