I think the problem is more likely to be that Larian does not expect the majority of their players to be genocidal psychopathic min-maxers, and they will aim the difficulty assuming players do NOT murder the entire map, which leads to the min-maxers being overlevelled.
That problem though, can be more easily solved by assigning experience based on the challenge of enemies, so level 1 peasants and rats offer chump change, and the level 4 Silent Monks 700 XP will also likely be toned down a bit because they do not offer much challenge because they're always fought 4 on 1.
