The riders number of first places are checked across all rounds of the series.
If they are the same then second places are test and in this way the first instance of a rider having a higher count will be awarded the higher place in the series.
If a rider moves up a grade then the number of current points can be decreased by 50%.
The logic is that as an example 50 points gained in D Grade is not the same as 50 points gained in C Grade.
This can be seen if you download the spread sheet which has a column for "Adjusted" which represents the points deducted.