Repro steps:
Accounts payable>Purchase orders>All purchase orders
Open the purchase order with invoice (different currencies)
Go to Journals>Invoice
Open Voucher -> General and we can see exchange rate as below.

This exchange rate is different from currency exchange rate for that specific date in the system. As it was noted: "This is a calculated field" and "the exchange rate that is displayed might differ from the exchange rate that is defined in the Currency exchange rates form."
May I know how this is calculated and if somebody knows why it is different?
Thank you all