I can't really make sense of the reading levels. I mapped them all out for a test query
Adds up nicely to 100%, so results should only ever appear in one of the categories right?
So the top 10 results with no reading level set mapped this way:
1 -> Appears in Basic + Intermediate at no.1 in both
2 -> No appearance in any category on the first page
3 -> Basic no.2
4 -> No appearance
5 -> Basic no.3
6 -> Basic no.4
7 -> Intermediate no.2
8 -> Basic no.5
9 -> No appearance
10 -> No appearance
11 -> No appearance (yes, there were 11 results)
What I'm most intrigued by is how #s 2,4,9,10 & 11 made an appearance in the main rankings without ranking in any of the reading level specific results.
Perhaps these are results where the content is less what it's judged on and inbound links are what's putting them in the top 10?
Results will vary for different terms too of course. I'm sure others are more slanted towards intermediate, etc.