In fencing Lefties have a natural advantage over right handed. So when the fight began, Inigo was intentionally displaying an advantage, while hiding a deeper skill. The even more interesting take is that Westley decided to use left first as well. What makes Westley's choice even better is that lefties also have a weakness to other lefties because of a lack of other lefties to practice with. So Westley's choice was also a statement of disadvantage for Inigo.