I think there are a few factors, such as how much of the book you actually read.

There isn't anything wrong with not finishing a book but if you only read like 30 pages out of 500, then I don't really think you can claim you "read" the book. That tends to suggest you either finished the book or read the majority of it.

I personally don't count them as read unless I read the whole thing. Otherwise they go under "tried to read" or "started to read"