The Customer open order amount is updated every time a sales order or service order is added, changed or deleted.   For sales orders, the open order amount does not include quotes and is decreased by the amount that has been invoiced.   Service Orders are different.   In a service order, the amounts on the totals page are for the complete order, including all the service item entries.   Since it is possible to invoice one service item entry without invoicing others, the totals for a service order might include amounts that have already been invoiced.   These amounts will still show up in the open order field.