I know it's comparing apples to oranges, but I contribute a lot on Stack Overflow so I took a look at the results.

https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

I was expecting a higher proportion in the "advanced" category.

For comparison:

Math Overflow: https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

OnStartups: https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

English Language & Usage: https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

CS Theory: https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

Seasoned Advice (Cooking): https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

Physics: https://encrypted.google.com/search?hl=en&tbs=rl%3A1&...

It's no surprise really, but it seems like the more technical jargon used on a site, the higher the reading level Google assigns.