This was my thought, as well.
The landscape has changed tremendously since 2009. Amazon has made it easy for anyone to self-publish any old thing. The one metric I want to see accounted for in these surveys, which never is accounted for, is output.
There are traditionally published writers who may only put out a book once every two or three years, and then there are self-published writers who are putting out a new book every two or three months.
And then there are self-published writers who released one book, didn’t find the instant massive success they thought they would, and never published anything again.
Failing to account for rate of output means that these surveys end up telling us very little.