I agree leaving with 1 HP is luck. But if you engage THINKING yo can kill a champion and that champion ends up with 1HP, he had as much luck as you would have had* if you had killed him.
So lets say I have 1753HP and a Darius (fuck you Darius) deals 1752 damage with his full combo, and I run spamming laugh. Yeah, I was lucky, and I escaped. Lets say now that he deals me 1753 damage. Now HE was lucky. So, according to everyone, its not possible to calculate the exact ammount of damage he would do (if you are human), so you can basically guess you MIGHT get the kill. You know you will deal about 1700-1800 damage and that you have a 50% chance of getting it. And you didnt. In a 50% chance. How is that the other player being lucky??????
I call it "You should have poked a little bit more so you would have been sure you could get that kill. But you took a risk, and you missed. No luck there".
* English is not my first languaje, so all this past conditionals might be completely wrong. ^^