Reviewer’s opinions of your book are not only subjective, but their rating systems are often inconsistent as well. I can’t believe how many 3-star reviews I’ve seen where the person goes on to say how good the book was. I don’t know about you, but when a reviewer awards less than 4-stars I expect to see some explanation for why the book rated such a mediocre review.
I suppose you can associate the 5-star system with academic grades. In other words:
***** = A
**** = B
*** = C
** = D
In that context, I suppose a 3-star review thus equates to a C, which supposedly is average. But I still find it confusing, if that’s the case, how a reviewer will then go on to compliment the book and say good things about it without saying how it could be improved.
As an author, you want reviews and should thus be reviewing books yourself. Bear this in mind when you do so: If you give a book a poor rating, do the author and readers of the review a favor and tell them why, even if it’s purely subjective, such as you didn’t like the main character or the writing style. That will give them a better idea what you’re trying to say.