A character's Mythic+ score is calculated by summing their highest scoring run from each dungeon. Characters earn a score for each of their roles, and a score across all of their roles combined.
You can see an example of this in the image below:
This character, Combatcow, had individual role scores of 319.60 for Tank, and 935.54 for DPS. However, their combined Mythic+ score is 1061.32. This illustrates that the "All Roles" score is not just a sum of the scores you have for each role. Instead, it represents the highest scoring runs you've completed for each dungeon, regardless of role. You can see all the dungeons that make up your score by viewing the "Scored Runs" tab.
Note: The list of runs contributing to your score are also displayed on the Characters Leaderboard.