only idea i have is that you killed him, but a mob had already taken 50% of his health. If this happens then you don't get credit for the kill. This is the only thought I have without knowing the situation.
My husband did that to a guy once in pvp4... he got an ammo global off of him for like 400 ped. The guy revives in the SC and starts hollering in all chat how he was a cheat and he was gonna turn him in bla bla bla.... I never laughed so hard in my life.