So, a single S-Rank hero is a S-Rank party on his own, but if she/he form a party with a capable B-Rank superior adventurer, they are a A-Rank party and considered weaker, than the Hero alone. Does make sense, except it doesn't. ^^
She should have added, that bigger parties are considered stronger, even with a lower rank, or that you need certain numbers of people in your party to take a quest or something.