With customer table on the top of indexing order, you will never be able to get the order as you want. Looking at your report, the system will display the result based on the customer and not on the sales header no. lets say if system gets first customer A, then the system will show all the invoices of customer A irrespective of document no.
Its is not mandatory that for a particular customer lets say ‘A’ the invoices made are in series. it may take no. as say 1,5,8,14 and so on. So when we apply the customer no. table on the top of the indent, the system will display all the records related to the Customer ‘A’ so you wont get the document no. 2,3,4 after 1. After one you will only be able to see 5,8,14.