Amount due calculates the total unpaid amount based on the unpaid/partly paid invoices.
As you can see in the "payments received" menu, there are payments registered on behalf of the client that are not associated with an invoice;
These payments have been calculated in client balance/statement, reducing balance. However they do not directly affect "amount due" as they are not associated with any invoice.
To do the above, you may click inside the entry and use it to pay off one or more invoices. Alternatively, you can go to "Invoices" and pay off documents one by one.