If you post book reviews you'll almost always find you're asked to post ratings too. On many sites, they won't even take the review til you give them a rating. Not a problem, perhaps, unless you happen to have a thing about numbers. And being some kind of mathematician still, I kind of do.
Back in school, in England, we got percentages, not letter-grades, or 1 to 5s. 100% was perfection, no room for improvement, and the only place you might get it was in math. In science, the experiment could've been just a bit more clearly described. In english, the essay may have no grammar errors, but couldn't you have added some more emotion? But in math, the answer's wrong or right, and even if the score's split between stages, each stage itself is either right or wrong. (Of course, in college I learned that even math can't reach perfection, but that's another tale.)
So now I give grades of 1-5 to books. Actually, I give grades of 3-5: 5 means I love it so much I can't stop telling everyone they ought to read it. 4 means I love it enough to recommend it to anyone who asks. 3 means I'll recommend it to some people, but not everyone. If everything gets five there's no room for improvement, no room for Wow, no room for meaning, at least for me.
So if you ask me to write a book review, please let me apologize in advance for the possibility that I'll rate it less than 5. It doesn't mean I don't love it; just that it didn't make me force my family, friends and neighbors to listen to me rabbiting on about it. And anyway, the writer in me says it's the words that count.