Surprised at the findings? The index is less about total funding (although, per capita, the U.S. is no world leader by that measure either), and more about how well aid dollars reach their beneficiaries.
Those principles, among other things, enshrine the goals of humanitarian aid as alleviating suffering according to need, irrespective of political goals, and in a way that supports long-term development.
For the U.S., a mediocre ranking reflects mixed performance. While funding is allocated relatively well along international guidelines, much of the country's aid is tightly earmarked for specific projects or comes as physical goods instead of cash.
It's bottom of the pack in implementing international humanitarian and human rights laws, having refused to ratify key international treaties. Survey responses also rank U.S. aid lowest in perceptions of "neutrality" and "independence" from political and strategic considerations.