Single shoulder plates are placed on the right arm, as the right arm is more vital than the left as your major arteries are located in your upper right torso.
Being wounded in the right shoulder in most cases can be very fatal, as puncturing an artery will not heal, and will bleed out very fast.
Well actually.... the differences is neglible
[SPOILER="Anatomy"]
[Image: http://upload.wikimedia.org/wikipedia/commons/1/10/Arterial_System_en.svg]
[/SPOILER]Perhaps the Sword-arm needs more protection? Assuming you're right handed. [S]Which I'm not[/S]