Hi Dilip
This behaviour is WAD (Working as Designed).
REF:
http://www-01.ibm.com/support/docview.wss?uid=swg21386003
You can indeed open a PMR requesting an Enhancement Request to have this behaviour modified to suit your needs i.e. bring all my attachments in Memo to top ,instead of looking into it who all has attached a document in the memo.
Kind Regards,
Paul