Smite can kill a ulting Trynd

I dont know if this is a known bug or if this is a but at all, but in my latest game I was playing against a trynd and i noticed that whenever he was about to die and ulted I smite him and it killed him. But it only killed him if he was about to die. I couldnt just smite him as soon as he ulted, he had to be at 1-10ish hp for it to kill him. This didnt win us the game (we still won, I dominated that game) but it was still fun to see him dive in almost die and i just smite kill him. If this is a bug i would enjoy it staying just to see all the . Btw I had the Skirmisher's sabre and have not tried it with the other one.