In the part relating to Smaug and his subsequent demise, it says "by more luck than wisdom Smaug was eventually slain..." and then describes Bard's arrow as "well-aimed". If it is well-aimed, it isn't luck; it's talent. There's a reason Bard is called "the Bowman". It's not like he just fired wildly and inadvertantly hit a square of missing scale, the weakness was known (wisdom) and exploited (talent).