Yeah this rating system is very odd. Looking at my story right now, if we add together all the scores, we get 183, then divided by the number of ratings (28) we get 6.53. My story's currently sitting on 5.9.
Meanwhile a random story I took for comparison has the sum of scores at 97, divided by the number of ratings (17) we get 5.7, yet the story's sitting on a 6.5 rating.
Is there even an approximate guess how it's being done?