The problem is that the Nightingale Blade is a leveled item. All items you craft and most items you find are unleveled and therefore scale with your level. Leveled items remain at the same level at which you found them. If you got the Nightingale Blade, it would do more damage than the Rune Axe which does 11 base damage.
https://www.reddit.com/r/skyrim/comments/n4jva1/someone_explain_this_to_me/



