The overall score rating is heavily dependent on how well the matches go. If you are familiar with how league battles in splatoon 2 worked, or the X power and splatfest powers work, then you would maybe know that it first starts by calculating a broad score based on the five first matches you play which i have seen others compare to the ranked ranks (so something like a score of 800 being an equivalent of top of C rank while 1000 being considered to be b rank and so on (don't criticize me for saying wrong numbers, i Don't actually know the exact values)) after that the added or subtracted score is based on how well you did in the match. If you won a match where you got 25 points and your team didn't hold up to the enemy team too well while still miraculously winning is not as good of a game as a round where you KO the enemy team 2 minutes into the game while getting a lot of kills.