At risk of slightly going off topic of this thread, @KateTriesAgain the reason it’s graded at level 33 is because people have graded 容疑者Xの献身 as level 33 and it’s in the same series. However, if the volumes of that particular series are not tightly correlated in difficulty, do let me know! We can chat more in this thread.
(PS: Welcome to the site!)