The only convincing answer can be: yup. It's a significant (and widespread) inconsistency created by:
- Gameplay considerations
- Developers (and players!) with no grasp of or interest in quasi-realism or fantastic reality
- Players (and developers!) with no knowledge or relevant experience of surviving harsh conditions, outdoor living, combat, training, physical challenge...
- This game series and setting in particular is somewhat cartoon-ish in its approach [to this sort of thing]
The same issue (if it's an issue) applies to any large monsters as much as it does the Dremora and their would-be martial might and ferocity; all else being allowed in reality, just picture what would happen to a 6' man in steel armour and sword in close combat with, let's say, a giant or a dragon. Result: screen splatter, say goodbye. Too physically superior to fight.
I roleplay my characters (the ones who're going to fight and win against things like Dremora, anyway) as mighty heroes, within the bounds of some kind of consistent quasi-reality but assuming the potential for physical prowess above and beyond what their size and weight would otherwise allow. I also (usually) play larger-than-life, immortal heroes [in TES, I guess they learn leaping-magicks as well], which puts them on roughly even footing with immortal war spirits, before taking prophecy/mythic roles/varliance into account. You know, the parry-arrows-in-flight, sprint-all-day, turn-the-tides-of-war sort of mighty hero. Takes a while(!) to get there, of course, which is traditionally the more satisfying part [of a story]...
Most people in my experience either don't roleplay (and so a goofy cloth-clad peasant warrior can mow down war spirits without a thought), or roleplay along the same lines as my bullet points a few paragraphs up - in line with TES' somewhat silly/cartoonish [is it too controversial to say "American"?] vision of fantasy.
So basically, these characters couldn't beat the average Dremora, or the average troll or giant spider either, and one arrow would be enough to kill or maim them. But the players largely don't care, and the setting as presented by the game supports and even encourages this way of thinking. Which of course is perfectly fine.