I have A LOT of reviews that I've left, as I read about 2-3 books a week, and out of lots of pages of reviews, they're all 5 star, except for maybe four or five and most of those are 4-star.
So the reviews I leave are 99% 5 stars. Why? Well, I'm an author. I know how bad it hurts us to get a bad review. I refuse to do that to my tribe. If I don't like the book, I keep it to myself. Just like in real life if I have nothing nice to say, I try to say nothing at all. That's just my personal feelings on it, and I completely understand readers need those low reviews (if they're warranted) so they can make a choice to buy or not. But as an author, I just can't leave them.
Regarding reviews on my own work. I have a crapload of REALLY long reviews. Usually from bloggers who received ARC's, and a crapload of short reviews. I assume those are the ones that Amazon prompts cuz some will say something like, "It was good." Or "Didn't like it." If I look at the other reviews that person left, they are much the same, because that is how that person reviews.
Nothing nefarious about that, however, if I can see how someone might think they're not real. But...they are