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 Use Retainer payment method. The Retainer Applied field only shows the retainer that is applied on the invoice itself. This is done by adding the retainer value in the Retainer field on the Invoice Batch screen when creating the invoice, or in the Subtotals view of the Time & Expenses tab while the invoice is still a draft. Check out this article for details: Applying retainers while invoicing.