Why user give low ratings to books they bought

In a book recommender system, user sometimes give low ratings to the
book they bought. Here, I supose there is no cheating in the system
and users are not forced to rate items.

A user buy a book because he/she like it. So, why he give low rating
to the book after he read it? This may because this book is not
satisfy this users interest. So, this user like most of the
properities of the book (he may like the topic, the author, or this
book is recommended by one of his best friend), if not, he will not
buy this book, but one of two disadvantage of this book may make he
give low rating to this book.

Above words seem like common sense. But we can draw one conclusion
from this example, if a user give high rating to a book, he may like
all of the properities the book, but if a user give low rating to a
book, he may only hate part of the book because if he hate all parts
of the book, he will never buy it.