I did and the best explanation I would go with is:
I think the rating is dependent on the publics reaction to the episode. If a show had 11 viewers but it was a very good show, it may be rated a 4.0. Where as if a show had 30 viewers but the episode was terrible, it may recieve a 2.5
Just because people are watching it, doesn't mean it's a great show.
Otherwise it appears they get the rating from an "advlt demo" meaning that maybe they screen it and get people to vote on the episode?

I'm not quite sure. I do know that when I was at Universal Studio's a few weeks ago I did a screening for 4 NBC shows and gave them feedback (All the shows were pretty lousy IMO). So other than those possibilities, I have no idea how they rate shows vs the amount of people watching.