[fnands]

US Americans is usually used outside the US to differentiate inhabitants of the US from citizens of other countries in the Americas (e.g. Mexico, Canada, and all of central and South America).

E.g. in Germany, inhabitants of the US are usually called "US amerikaner", so this is a direct translation (the Kiel institute is in Germany).

It's just Germans being stereotypically precise about things (and reminding citizens of the USA that they aren't the only people in the Americas).