AME Invoice Approval

We are currently using invoice approval workflow.  AME is currently installed and set up, but we have discovered an issue.  Basically, we want Inv Appvl Workflow to perform as follows:

All invoices except for PO matched and unmatched invoices for vendors that are of a certain "vendor type" must be approved via the approval hierarchy.  Seems simple enough.

However, right now, if the Requester field is empty in the invoice workbench (for matched or unmatched invoices), and if the user initiates approval, the invoice is going to a status of Approval Not Required.  My question is, is there some sort of rule causing this to happen?  You would think that the logic would say that when you check "Use Invoice Approval Workflow" in the Payables Options, that this means ALL invoices require approval.  You then use AME to enter exceptions, etc., such as if the Requester field is blank, then approval goes to Not Required.   I'm thinking that