With patch 9.1 Blizzard has introduced their own mythic plus rating system. As a result, Raider IO will be adopting their scoring system so things remain easy to compare and reduce any confusion.
If you wish to read about the exact way the score is calculated, please check out this wowhead article on it:
https://www.wowhead.com/news/how-blizzards-mythic-rating-works-in-game-mythic-score-explained-in-patch-9-1-323046
You can read about the changes this will bring to the site and addon here:
https://www.wowhead.com/news/how-blizzards-mythic-rating-works-in-game-mythic-score-explained-in-patch-9-1-323046