It's much easier to quantify some kind of sword skill than luck. What IS luck anyway? Would you say that someone that wins the jackpot at a casino on his first coin is luckier than someone that sink 1000 of $ and got nothing back? What is the "lucky" guy got crushed by a falling piano while running to announce the good news to his family? Was that luck or not luck? What about the consequences of some "lucky" roll for next day? Next week?
The flaw in your argument is that, yes, sword ability was determined by a SKILL number. But Luck was determined by an ATTRIBUTE number. A skill, meaning how good one is at something, which I agree wouldn't make sense with Luck. However, an attribute refers to how much one has of something, such as Strength, Intelligence, Endurance, or in this case, Luck.