It's probably how VW got a Euro spec engine to meet US regulations. Euro specs measure pollution per distance - the way to win is to burn fuel really efficiently. US specs measure pollution per unit of fuel consumption - the way to win is to burn fuel really cleanly. That difference is a big reason why they have much more fuel efficient vehicles in Europe. It's much easier to get a larger engine to burn cleaner. Most manufacturers that sell the same engine in both continents use different tunings in each, where the EU one gets better fuel economy and the US one burns cleaner.
Manufacturers have been trying to bring the incredible economy that small diesels in Europe get to the US for years but it turns out making a diesel that is significantly more efficient than a comparable gas engine and also meets EPA regulations is really hard. For example, Mazda has been promising Skyactiv-D (diesel) engines in the US for years now, but they keep getting delayed because they're not satisfied with their performance.