Why are volunteer expenses getting created as Standard Invoices rather than Payment Requests in AP

We have some users setup as Volunteers in the system. When the volunteers raise expenses, these expenses are approved as normal in the Expenses Work area and processed to Invoices. These Invoices come in with the invoice Type as 'Standard Invoice' rather than as 'Payment Requests' and hence require approval. There are no approval rules setup in the system for expense reports hence these are stuck in the approval workflow as 'Initiated'.

Are volunteer expenses different from Normal Employee or Contingent Worker expenses ?