Because we all have our own sliding scale of what matters! I've completed the Pinside exercise of weighting what matters most to me (not an easy task, but certainly doable without too much second thinking) and my ratings are 100% based on that. Besides the fact that we can all weight things differently, It's obvious that alot of people are not rating things honestly in their own ratings.
Example...my HONEST rating of ACDC Premium is around 8.2. For things like Fun, Lastability, Rules, Lighting, Music and Dots...I gave it a 6 out of 6. EXCELLENT. But there is no freaking way I am going to give it any more than a "decent" (that's as low as I went on ACDC) for it's playfield art, cab art, backglass, layout, etc. THEREFORE, it isn't going to rate as high as a lot of other games that I may not like as much. But that's OK, because there are a number of factors that enter into a games popularity.
So, in conclusion, Pinside ratings don't matter for two reasons:
1) everyone's rating is weighted differently
2) people are lying
***I'd love to see actual ratings that are based on Pinsides rating system...but with HONEST answers and where everything is weighted equally. I think that would be really interesting.