How can "amount due" be different to a client's balance in their statement?

How can "amount due" be different to a client's balance in their statement?

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 the balance. However, they do not directly affect the "amount due" as they are not associated with any invoice and are shown as "Excess amount".

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.