"[W]hile the explicit compensation requirement of the [Impartial Conduct Standards] is that advisers and Financial Institutions cannot receive more than reasonable compensation, the DOL is saying that a Financial Institution's compensation structures cannot promote investment recommendations that are not in the best interest of the investor.... One possible interpretation is that, even though the compensation of the adviser can vary, both for similar products (e.g., mutual funds) and among product categories (e.g., mutual funds vs. variable annuities), the variation cannot be so great as to unreasonably promote advice that is inconsistent with the best interest standard of care."