Project budgetary control -- available fund calculation

Dear Experts ,

       I have one question for project budget available fund calculation .

       I know in project , internet expense report no fund checking function . But ! why the expense report amount is not calcuated to "Actual Amount"  and deduct from funds ?

       for example . if I have one expense report amount = USD 1000  and internet expense report is approved and export into Payables and Project Costing module . On project status inquiry function , it was collected to Actual amount . However when I create another payables invoice USD 700 and process fund checking . It's passed . WHY ? my available fund should only 500 . and through Budget history fund checking result , it does not include expense report 1000  in actual amount .( logic is different from project status inquiry )