Invoice_Amount not equal to Approval_Amount on force approved invoice

We are seeing an issue with invoice_amount not equalling the approval_amount in the ap invoices all table. Invoices are entered and force approved.  When we query the ap invoices all table we are getting a different invoice amount than approval amount.  I have verified this with multiple systems not specific to one client.  Why is the invoice amount showing a different total than the approval amount?  Where does the approval amount come from?  In one of the cases the invoice amount is $419.  The approval amount for the invoice shows $22,500.  This is not what I expected to see, and