There's your total score and your ranked score.
Total score will increase from everything regardless what you play (iirc it doesn't even matter if you finish the map or not).
Ranked score is added up from your top scores on every map you played.
That means if you retry the same map a lot, you will gain total score but little to none ranked score.
In multiplayer you're usually setting new scores on maps you didn't have a score before which means that most things you play in multi adds to your ranked score.