Outgoing payment post different LC amount for same exchange rate as AP INV

Hi All,

I have this entry in my out going payments linked to a AP down payment that I cannot explain. This is a foreign currency transaction

1. AP down payment

     FC

     USD 381,000 @ document exchange rate of .931277

     LC

     AUD 409115.66

2. outgoing payment

     FC

     USD 381,000 @ document exchange rate of .931277

     LC

     AUD 448,235.29

     Although the outgoing payment document exchange rate is same as AP down payment , SAP has applied rate from exchange rate table (@.85)

as a result LC amount is different and its created an exchange gain/loss entry. Any idea why this happen?

Other info

on payment means screen all transactions in USD and payment is NOT on account.

Thanks