What led me to this conclusion was a couple of reviews I received recently. Frankly both of the written parts were great, however, the first one gave me 5 stars and the second 3 stars, even though the second one said specifically that they were looking forward to more of the series.
Hence, I think what it comes down to is interpretation.
When you have your own blog a star rating makes sense because everything on your blog is rated by you using the same criteria. On a site like Amazon or Smashwords the ratings are given by different people using different criteria.
Some people give five stars if they enjoyed a book. Other people only give five stars if they think it is the most exceptional title ever.
Personally if I really liked the book I give 5* and if I liked it, but it had some problems I give it 4*.
It doesn't even matter if there are suggestions as to what the number of stars mean because mostly people don't read that part, they go with what's in their own head.
Add to that the fact that on your own blog you usually have an explanation of the star rating in the side bar or something and on a site like Amazon some people only go for 5* books and others actually look at the reviews, and the mire just gets deeper. It kind of makes sense that some authors were faking their stars reviews because it was the only way to guarantee a result; it wasn't fair or right, but it was logical.
I'm much more likely to be swayed by the cover or the blurb than a star rating. I only really take note of star values on Amazon etc if they are 1 or 2 stars and not just 1 or 2 reviews.
When it comes to DVDs I don't even ever look at the star rating because I often disagree with all the reviewers anyway :).