Throw out the best and the worst, and evaluate the rest.
When I first started in this wonderful life of ours, I rated very high. Two reasons, first is that the women I saw were outstanding compared to the women that I could attract IRL and that's still the case, but more importantly some of my first providers were and still are the most beautiful women I've ever seen IRL, much less sucked and fucked.
Most of my reviews have high numbers and there's a good reason for that, I do my homework, I read reviews, I ensure her menu includes my likes, so there are very few less than stellar experiences.
The bottom line though for the number system is that it's subjective, what I find worthy of a 10 score is likely different than what you do. I saw a woman who has dyed her hair bright red, I thought prior to seeing her that the unnatural look of that would be a deduction of one number, and then I spent time with her and it fits her and her personality and it actually added to her sexiness.
Beauty IS in the eye of the beholder, and performance is just as subjective. Read the reviews, pay little attention to the numbers.