Last hitting champions/minions off.

I have noticed within the last 3-4 days that last hitting champions that I should have gotten the kill on when I did more damage than they have health (this is team fights and I know I had last hit due to having hit the champ with more damage than they have health). I have not received the kill but it goes to someone else. This has been consistent through the week and it has happened with champion kills and jungle minions. A few of my friends have noticed this as well and are confused if this is deliberate or if this is a bug. It is starting to get a little ridiculous when this happens often.