it is based on the duration you select, as you set monthly.
For Example:
If player A placed 900 bet in Live Casino on September.
So you can see player A is in the list with turnover 900 when you select September rebate.
In the following month, October, player A placed 500 bet in live casino.
So you can see player A is in the list with 500 turnover when you select October rebate.
Hope this helps. Please remember that if you set monthly, you can only submit your rebate for player ONCE in every month