It's just that people frequently don't bother reviewing anything, and when people finished reading a book it is usually a 4 or 5 rating. Most people that wouldn't like it will either drop the book based on synopsis/genre/reviewers and never rate or DNF early on and never bother to rate it.

I am not sure what causes people to read 3.5 rated books. Is it only when a lot of people read them just cause its popular? Author mentioned some about "well reviewed books".