I think it might be similar to how the skill level works in battlefield. They probably take into consideration the skill level of the person you died against and or killed, and maybe even the weapons that squared off too. For example in a head to head with Suros Regime vs Shingen, the Regime will win every time when skill isn't involved. So, your friend may have a higher combat rating due to them maybe killing someone with higher rating or a stronger weapon. Just a hunch