Hello Ahmed,
Yes this is correct behavior as there is exchange rate difference it will post to exchange difference account (KDM) as you defined in OBYC.
And as far as Group currency and Local currency is concerned it may mismatch based on Your setting in OB22. I am assuming this might be the difference system might have followed below step based on your settings.
USD=AED ( Local currency)
AED=USD ( Group currency)
In Your Case 1st local Currency ( Converts based on Document Currency)
AED/USD---0.27 ( As per Document Currency or Transactional currency)
Transaction Posted is AED1000
270USD=AED1000
1st conversion is USD to AED
Then 2nd Local ( group) currency ( Converts based on Local Currency if option 2)
USD/AED 3.6729
AED1000=USD272.26
2nd conversion is based on Local currency
In short maintain option 1 for source currency Transaction t-Code OB22 in one to avoid such differences.
Best regards
Hrusikesh