How rating works
Rating for anime and manga works the same way. It makes an average score with all the user's scores. To have a general score it needs to have at least 10 users score.
The score is not updated every time a new score is added or edited. It's updated programmatically every hour but it's limited in time, so the timing depends on the number of anime in the database. The order of update is based on the last score update so the first anime or manga score to be updated it's the one that needs it the most.