When Projection Method is set to Sales Forecast when you process Transactions>Demand Planning>Calculate Item Demand Plan, the system uses the Probability and not the Status of an Opportunity. If the Probability is more than 0% then this transaction will be considered.
Therefore, if the Status is Closed Lost but the Probability is 1% then the system will still include this in the Demand Plan calculation. Note that ideally when you set Status to Closed Lost the Probability is assigned to 0% but users can still override this field. Also when you use CSV Import to update an Opportunity and you only map the Status, the system does not automatically update the Probability field to 0%.