The app stores do not provide individual ratings, we take a snapshot of the star ratings count each day. The reviewer or the app store can remove ratings at any time which can cause some fluctuations in the ratings numbers.
The Ratings over Time average is calculated for the selected period only (1). The page shows the total number of star ratings for the period (2) and the average star rating for the selected period is also shown (3).
We calculate the star counts for a given period by taking the difference in the star counts from the first day of the selected period to the last day (as shown below).
We then use these star counts to calculate the average for the selected period.