My writings often reach 6 and 7. Why? Am I using preposterously extraordinary words? No, as a third language English speaker, my texts are quite newbie-ish. The thing that makes the "readability level" jump sky high for me is my tradition of writing long sentences that go on and on, without ever stopping, because for some reason I prefer commas over periods, even if it means a sentence that's half a page long.
Then again, I have some short stories with lots of sentence fragments in order to confuse the reader, and it is read as far easier.

I don't consider novels with longer sentences harder to read. Same goes for words with more than two syllables. What makes a novel harder for me is vocabulary and paragraph construction. Even if you use only one and two syllable words, with 5 word long sentences, a paragraph that's a page long creeps me out. And that's what usually isn't measured.

I once found a site that checks your novel title and displays "the chance you have for it to be successful". Now that was something that made me pull out my hair.