Dungeon achievement bug

(It has probably already been reported, but never fixed)
In Achievements - Activities - Dungeons, you’re supposed to earn achievements after completing dungeons at e1 e5 e8 e17 difficulties.
- you get the e5 achievs after completing e8 diff
- you get the e8 achievs after completing e11 diff

So, is it a bug, or just a poorly written description ?