Exp Report Skips Prepay Audit

We have Expense Report approvals set up to go first to the HR Supervisor and then to a Prepay Auditor before it is Approved for Payment.  When an expense report is approved by an HR Supervisor who is also set up as a Prepay Auditor for the same BU/OU combination it will skip the Prepay Audit step and go straight to Approved for Payment.  There are multiple Prepay Auditors set up for the BU/OU combination so shouldn't the report still pool for an audit?