It specifically said that the sword granted "leadership".

I suppose you could assume that it granted Parson some tactical abilities, that would help him lead troops, but I don't really think so. Not only is he already excellent in that respect, (from what we've seen), but the box implied that the prizes were to make up for deficiencies in Parson that stopped him from being the Perfect Warlord.

He had a low leadership value, that isn't perfect. Thus, the spell fixed this, by giving him a sword that increased it. It's perfectly logical.