Well... I think its based upon on how long you battled the creature and how much experience you got. When I was playing BG2:SoA
(Baldur's Gate 2: Shadows of Amn), I killed a Red Dragon and it was my most powerful monster beaten since then. In Icewind Dale 2, I killed two White/Silver Dragons in the River Caves and it was my most powerful enemy vanquished since then.
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> AM I LONG-WINDED OR WHAT?
'The Child of Bhaal has awoken.....'
-Joneleth Irenicus