User had applied some client retainer on an invoice, but this is not showing in the Retainer Applied field on the Client Transactions report.
This can happen if you apply the retainer after creating the invoice using the User Retainer payment method. The Retainer Applied field only shows the retainer that is applied on the invoice during its creation. This is done by adding the retainer value in the Retainer field on the Create Batch Invoice screen when creating the invoice. Check out this article for details: Applying retainer while creating invoices.