Problems with acquiring the Overkill achievement on PS4

There is a problem with unlocking the overkill achievement on the PlayStation 4 minecraft, I have found that I currently own a diamond sword with smite 4 which for the achievement you need to deal nine hearts of damage to a mob or player to acquire this achievement, well I had found out that skeletons and zombies both share 10 hearts of health which concerns me as I deal over nine hearts with my diamond sword and my strength two potion I can kill a zombie with one hit which means I have dealt over 9 hearts of damage and I have not yet acquired the Overkill achievement is there anything, I am not sure weather this is a bug or I'm doing something wrong here please let me know, thank you Jack