My company performs professional statistical and quantitative analysis for the largest international corporations at the highest level. Our company website ranks:
17% Basic
75% Intermediate
6% Advanced
I would speculate based on this that Google's reading-level technology works by breaking down individual elements of pages, whether posts or otherwise ... and probably what triggers the "advanced" feature is the number of words that are used beyond a certain threshold of commonality in the English lexicon. In our case, pages that describe the science behind some of what we do, using advanced mapping or stats terms -- like an entire article about chloropleth cartograms -- probably are the only ones that are catalogued as advanced. However, pages that generally engage in intelligent discussion are "intermediate" ... and we have an entire blog that discusses issues in this vein. Those pages just with simple instructions on how to download a video, etc, are "basic."
Probably as the technology advances it will be able to figure out what the overall content is like at a website ... it's not that relevant, after all, if some instructions somewhere use plain English if the majority of substantive pages are actually complex.
Take mit.edu as an example:
25% Basic
36% Intermediate
38% Advanced
Obviously at MIT there are a lot of public-facing pages that present images of the school and simple instructions or features (like browsing a student directory). There are relatively fewer normal adult-level articles ... since that is an educational website and not a media outlet ... so there are fewer intermediate pages. But there are a lot more advanced pages, which no doubt are all of the pages dedicated to specific departments with published reports, etc, on advanced topics that penetrate into profound levels of scientific detail.
