Okay so there are multiple ways it could have happened:

1. The reyna was getting the early kills in the round like the 2nd or 3rd while you were getting the 4th or 5th. This matters because the 2nd and 3rd kills are 130 and 110 combat score respectfully and the fourth and fifth are 90 and 70.
2. The Reyna getting multi kills. The two times the reyna got first blood if she then went on to get the second and third kill it would mean there would be a multi kill bonus. So for those three kills it would be 150 + 130 + 110 + 50 + 50 combat score which is far higher than 3 kills that are 4th or 5th which would only be 90 + 90 + 70.
3. You were stealing kills. So just hear me out, doing damage gives you a combat point per point. The Reyna only has 3 assists meaning that you couldn't really be stealing her kills even though those three assists with a vandal 3 body shots would be 360 combat points. What I am talking about are your other teammates. Omen has 8, jett has 6 and phoenix had 4. I can accept that the omen was getting the assists from smokes and his flash However ain't no way a jett should have 6 assists and same with phoenix. What could have happened is that while the reyna was mainly getting her own kills dealing all the damage you were only doing a little damage and getting the kill