Running Totals don't work correctly in Headers (Summary functions do but running totals don't). It must be giving you the Calculation that you are looking for but I am not sure why. Do the values change if you move it to GF3b?
Also if the InvoiceName is NULL are there still values that are totaled here? If so, try changing the RT formula to exclude these in the total:
(isnull({table.AccountName})=false or {table.AccountName}<>"") and {tblInvoiceDetail.Type} = ["S", "O", "M", "A"]