5* it may say...but is it the real deal?

My thoughts have turned back to ratings and comments today, this time not in relation to TripAdvisor and the fun I had with Mr Unhappy Bunny, but in relation to writing.

My question is simply this:
When it comes to books (especially ebooks), how much can I trust the rating that they have?
Maybe not as much as I should be able to, it seems.
I've downloaded quite a few ebooks recently, many of them with rating scores well above 4 and a plethora of enthusiastic 5* reviews waxing lyrical about great characters, fantastic plots, subtle unforeseen twists, etc.
Sadly, very few of them live up to the high expectations which these comments create...which is sad, because the stories are, on the whole, not bad at all. They are admirably written, engaging and well-crafted...unfortunately, they just aren't as fantastically brilliant, witty, inventive and challenging as they are made out to be.
Why is this?
Is it me?
Maybe it is.
Maybe I just expect too much...or do I?
You see, I do have a benchmark of sorts, and that benchmark is the average rating of some of the greatest books of all time...

• 1984 (Goodreads average - 4.06)
• Ring of Bright Water (4.27)
• Flowers for Algernon (3.91)
• Lord of the Rings (4.41)
• Day of the Triffids (3.95)
• Hitchhikers Guide to the Galaxy (4.14)

...given these ratings, surely it is only reasonable to expect that anything with an average up around 4.00 will be fantastic...isn't it?