You might be right tcm... but statistics based on available info is all we've got to go on
I don't know why you're working out a percentage. AW reviews are almost meaningless and as tcm has said there can be many reasons for their varying numbers of feedback.
Pretty much the only stat I'd be interested in would be who has the highest total number of positive UKP reviews. After that perhaps their % positives compared to neutrals and negatives.
You could also work out "average positive reviews per month they've been on AW" to help highlight the new girls who haven't been working long but are seemingly doing very well but obviously there's limitations to the reliability of that (e.g. only been working a month and 1 positive review).
Still, it's the total raw numbers on here that give you the best indication (and of course it's still not a guarantee).