A user at Battlefields Forums says that:
I think it may be how you as a Player/Team did in attacking/defending. The number on the right I'm thinking is the average a player/Team should be at. The number on the left reflects how you/the Team did. The higher the value on the left compared to the right, the higher the (X) value will be that raises your ending score.
You said that your team failed at attacking thus matches the description above where the number on the lest is really low. You might have done really well at defense on the other hand thus the high defense score. Of course, sometimes it's not that accurate according to a Reddit post (the comments may provide useful):
According to another Reddit post, the number on the left is your rating and the number on the right is the standard set by the developers:
Last time this was asked I said it's probably a way to determine if your team did well compared to a "standard", in this case "27". God knows why, everyone knows 42 is the answer to everything but there you have it.
If your team's K/D, time needed to capture points on attack, time needed to kill off all attackers as defenders, etc.. is better than an average set by the devs, you get a better score multiplier after the round.
You can still lose and get a positive multiplier because of this.
The negative score you sometimes get is probably due to the fact that it's calculated against the defence standard when you are on the attacking force. I'm no programmer though so don't hold it against me if I'm wrong.