Sometimes it can be Athenas Sword, if they have the red halo over their head with Athenas healing ability they will gain life and instantly get back up. I noticed people doing that to me and once I used Athenas Sword I noticed why they got up so quickly at times.
but yea I can see the game getting confused at times.
I know but that's never the case when I brutal kill someone. They have no green glow.
Bump . Still not enough players do this, and I dont believe they mention this in the guide. Save your brothers!
They mention it briefly here http://www.godofwar.com/en-us/modes/favorofthegods/ although I think it would've been more helpful in the "Basics" section of the guide.