"Reading Level" categorizes the search results according to the level of expertise needed in reading the page. Simple and easy-to-comprehend pages are categorized as "Basic", where the advanced and high-standard pages are categorized "Advanced". "Intermediate" stays in between. Highly technical or articles with complex language structures are often categorized as "Advanced".
I searched "Kathiravelu Pradeeban" using Reading Level, and found the below.
Similarly, searched a few projects that I am interested in.
Basic - 14%
Advanced - 3%
Basic < 1%
It is pretty reasonable to have the highly complex and technically advanced OGSA-DAI to have more pages classified as "Advanced" and only a tiny bit classified as "Basic". However, AbiWord, the word processor has more contennt classified as "Intermediate". This again is reasonable for an end-reason software product. Google Reading Level is surely an interesting feature.