I have played many story matches. I am not sure how command rank is being calculated. I have had matches where I had the high score by double my next team mate and received 250 command ranks. Recently I played as Reyna, and received more than 800 points to spite having a much lower overall point total. Can we see you math please?
It’s based on the XP you get. Completing challenges is the best way to gain XP in my experience.
As Reliikki said you get additional XP when challenges are completed during the game. You prob had 1 or 2 completed and didn’t even realize it. It pops up on the top left side of the screen but hard to notice with all of the other things going on visually.