I've read the book based on the survey and "top earner" is any one on the survey that answered Yes to a question indicating whether you do, or could if you wanted, to live off your writing income.
As another poster said, "how you analyze the data" is where things get tricky as everyone sees something different, and Taleist won't release their "raw data" just the conclusions.
There is another survey going on for "all authors" (even unpublished ones) and that data will be made available to anyone who wants it.
Currently it has around 600 participants and they want to get to the size of Taleist so that means at least another 400 - so if you are a writer.....