Changing "Additional Reporting Currency"

When I change the “Additional Reporting Currency” in “General Ledger Setup”, Navision Financials (re-)calculates “Additional-Currency Amount” on the “G/L-entries”. The currency-exchange-rate used for this calculation seems to be the rate associated with “WORKDATE” and not “posting date” from the G/L-entry. Why is that? If you set the “Additional Reporting Currency” before posting to “G/L-entry”, then the rate from “Posting date” is used! If You change the “Additional Reporting Currency” to something else and then back to the original, you might end up with a different “Additional-Currency Amount” on the “G/L-entries”. Frank