Simple answer.
The game is getting harder.
Well, my advice is to keep the difficulty setting in normal. No it's not cheating, your characters gain max hp when they get a level (so if your fighter rolled 1 and he gained 3 hp you don't need to load game) , your wizard does not make any mistakes when writing scrolls to her book (again, you dont reload game when you lose a 9th level spell) and your characters cant permaently die (its not important for me when they die I load again)
In IWD, you get more XP if the difficulty is high. In BG2, it's just for to satify your ego
no xp penealty or reward but it's much better.
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"A silent blade does not cut always best..
A tender touch from the heart may heal the wounds of my friends..
Or a song from the heart..
A duet with nymphs..
Forget 'bout nymphs, let's play KISS!!"
-Yngwie J. Amadeus Zoltan