This is a super old thread (2 years)... but main problem is that when people buy a pin, they want to feel good about it, and will rarely say (I just spent $10k on a new pin, and it's only a 6), so even if it sucks, they convince themselves its awesome. In addition, as you learn the ruleset of a pin, it becomes much more enjoyable, so people who own pins, are going to enjoy those more than one they only play 5-6 games on.
I think that's why some of the older rare but awesome pins have lower ratings, because since there were only 1000 or so made, most people will have played them on location somewhere, and not gotten the full experience or even played one that wasn't dialed in right (weak flippers, balls getting stuck etc etc).
So I always read the reviews themselves to get context. See what people like and don't like on the pin, and maybe watch some videos of it playing if there are none near-by to play. I make my decision based on that. I've never really paid too much attention to the ranking, and have owned and enjoyed games that are in the 200's better than others that are in the top 50.