Sales order consuming forecast for quantity larger than Sales Order quantity

Summary:

We had loaded one External Forecast for a customer A and selected Ship to Consumption level as 'Customer' for that Demand Schedule in Plan Options.

Created a sales order for the same Customer A for same qty as externally loaded forecast qty and was expecting it to consume the externally loaded forecast only. But in addition to consuming this external forecast quantity equal to the Sales Order quantity, the sales order also consumed system generated forecasts (common for all the customers) of some more quantity.

Why did the Sales Order consume this additional quantity from the system generated forecast? Shouldn't the consumption be not more than the Sales Order quantity?