Invoice Amount Calculation on Agreement Life Cycle

Summary:

Does anyone know how the invoice amount on the Agreement Life Cycle is calculated?

Content (please ensure you mask any confidential information):

I am using the following query, which works in 99% of the cases. but in one BPA (example only), the amount queried is incorrect - it's lower than what shows on the page.

select sum(aida.amount) as Invoiced_Amount

from 

  po_headers_all pha

  ,po_lines_all pla

  ,po_distributions_all pda

  ,ap_invoice_distributions_all aida

  ,ap_invoices_all aia

where pha.po_header_id = pla.po_header_id

and pha.po_header_id = pda.po_header_id