The blog-owner was so kind as to answer, when I asked: "...both words and sentence structure (not random)."
I also found his comment on the original post for the app.: "Currently it analyzes vocabulary (use of words), number of words, commas, and semicolons in sentences, number of sentences with quotation marks and dashes (direct speech)."
So, now we know a little more. He promised to reveal more in upcoming blog posts... :)
I posted a fairly long part of the fanfic that I'm currently working on, and again ended up with Dan Brown... :) Maybe I should go dig out his books again, to see what's similar... *ponders*