How is expense_current_approver_id populated in ap_expense_report_headers_all table?

Does anyone know how the expense_current_approver_id field is populated in ap_expense_report_headers_all table?

I see the field is populated once the expense report is submitted. For some reason I cannot determine, the field is being populated by a person_id  that is not the override_approver_id for only certain employees. The override_approver_id is direct supervisor and has enough limit to approve these reports. If I could determine how this field is populated, I'm hoping I can find root cause for behavior. Also, we are using AME.