Well, it's a matter of mechanics in my opinion. The player and an NPC with the same weapon, same skill, perks and level will do the same damage on the default setting. On harder settings the formula is changed. This is why I call it false difficulty because the rules are being changed. The NPC's are not playing by the same rules as the player. It's like cheating ai in a sense.
I see, and I can understand why you would want to experience combat where enemies that are the same level as you being about equal to the PC.
However, in my experience when I played very briefly on Adept at around levels 1 - 5, I was able to easily kill every enemy I encountered, often in one or two blows fighting with crappy weapons or the novice level Flames spell with no perks. The game felt broken in a way because the combat encounters were suddenly boring. As soon as I figured out it was because my difficulty settings had changed to Adept when installing Patch 1.6, I quickly went back to Master.
Perhaps this has to do with the way encounter zones are designed, so you encounter many enemies that are lower level than the PC and only a few that are at the PC's level. Or perhaps it's simply being overly accustomed to dying a lot more frequently.
Incidentally, Duke's mod for Oblivion was also my favorite

With that mod, plus Combat Archery and FCOM, and the difficulty slider set to 75%, I died and reloaded probably 10 - 20 times for nearly every combat encounter. Suddenly I had to approach every possible encounter very cautiously, hiding from enemy archers behind trees, etc.