Sometimes a contact who was already closed as a Customer may be pushed back up the funnel for one reason or another. You may wish to manually edit the Lifecycle Stage property value within the contact record back to Lead or Sales Qualified Lead.
By default, the Sources report recognizes Customers by adding a tally in the Customer counter column. So what will happen in the Sources report for those contacts who were already counted as a Customer then changed back to a Lifecycle Stage further up the funnel? Once the Sources report's delay is caught up, the Customer counter will remove that contact's tally.