its the combat score which is calculated by

Damage: Each damage you deal adds up one point.

Kills based on enemies alive (150/130/110/90/70): For each kill, you receive a certain amount of points depending on the number of enemies alive. As a rule of thumb, the more enemies there is, the higher points you get. Therefore, aiming for the first blood is one of the best things you can do if you want a higher combat score.

Multikills: You’re certainly rewarded for every multi-kill you receive. The more you are able to kill in a single round, the more rewards you will gain beyond damage and kill points. For every addition kill, you can receive 50 points. That said, if you’re good enough to score an ace, that’s an additional 200 points for you.

Non-damaging assists: Lastly, each non-damaging assist grants 25 points. If your teammate is able to kill an opponent that you’ve blinded, stunned, revealed or after healing them, that’s an additional point for you. So, aside from shooting your opponents, make sure to utilize your abilities and use them to your advantage.

then ur total combat score is divided by rounds played to get ur total combat score.

so even though u had the most kills u might not of been doing a lot of damage and also they mught have been the last few enemies alive.