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, Gravykin, had individual role scores of 225.7 for Healer, and 1137.5 for DPS. However, their combined Mythic+ score is 1137.5. 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 "All Runs" tab.
You can also look at more specific runs by selecting the "Runs" tab (which defaults to "All Runs") and choosing "Timed Runs", which shows only the runs that were completed in time.
Note: The list of runs contributing to your score are also displayed on the Characters Leaderboard.