And on the other skill, Book of Five Rings is a skill that lets him hold his own against NPs [i]provided he has prep time.[/i]
It ties into his real-life historical habit of exploiting weaknesses in enemy techniques and weapons, and also he wrote a book of how if he caught you doing this he'd kick your ass.

His combat stats being higher than Kojiro's makes sense because, historically, he beat Kojiro in two hits with a stick.
The high strength especially makes sense. Katanas are heavy swords, and Musashi used his one-handed to develop his dual-wielding style. Moreover, he's historically described as being physically very large. I recall his height being listed as something like six feet, which was tall for a Westerner, let alone a Japanese.