I’m reading まほり 上 Level 30?? , which hasn’t been graded by anyone else yet, and which is definitely harder than Level 30.
The same author also wrote 図書館の魔女 第一巻 Level 37, and, subjectively, I think that level 37 is probably a lot closer than level 30 for まほり as well.
This is an incredibly minor thing, but: does it make sense that the estimate for an ungraded book’s level should default to an average based on that same author’s graded books? It won’t always be correct - there are authors who write both for children and adults, and 森見 登美彦’s books go all the way from level 27 to level 42 - but most authors tend to be consistently right around the same level of difficulty.