I got a generic response from Netsuite (suiteanswer 43346)
Rates may be affected by time intervals when they are captured and then uploaded to NetSuite. Real time exchange rates are never exact inverses because market rates for currency are directional. NetSuite loads rates specifically in the direction needed for the system and never auto calculates inverses. If you always want the exchange rate inverse to be 1÷ the direct rate, you can manually override the rate.