You have to keep in mind some companies ask for reviews and others don't.
Asking for reviews directs all the pleased customers to review you and not asking, chances are more of the ones not satisfied are going to seek out a place to voice their opinion which is only fair. A perfect example of this from a company we all know:
http://www.resellerratings.com/seller1867.html
People have varying opinions of Dell, but most surely know their customer satisfaction isn't as low as it appears in that link or they would not survive. If Dell asked their customers for reviews you would see their review numbers in the many hundreds per month and their scores I'm guessing double+.
You can bet most all of the high rated companies on here that do high numbers of reviews all ask for their reviews. You can also bet most of these companies make lists of who to ask and who not to ask depending on how their order went (how long it took to ship, if there were any problems).
Just a few things to keep in mind when looking at reviews.