Missing Price Rate...in the past

Hi all!

i've a weird situation where i created a sales order (dates from 10 to 30 sept. 2018, speaking about creation, delivery, price and so on) and all works fine, price is calculated (in AUD while the company currency is EUR but we mantained OB08 since 1st of July, 2018).
Then i update one item's Account Assignement and i insert a WBS as account assignement.
And here i got message SG105

Enter rate AUD / EUR rate type M for 01.01.2015 in the system settings
Message no. SG105
Diagnosis
For the conversion of an amount into another currency, an entry is missing in the currency conversion table.
Procedure
Add the missing entry in the currency conversion table.

i can understand the system needs the exchange rate, but i cannot understand why for 01.01.2015!
i have no date previous half july 2018 in both Sales Order and WBS.
i checked a lot of similar questions and i see in OB08 the rates are ok.

To make a test, i insert the row in OB08 at 01.01.2015 and it works.

The question is.. why?