The plausible reasons why the dutch are on average significantly taller than Americans are:
Dutch have a more wealth distribution.
And secondly the living and eating style, Dutch people eat better and lead a healthier lifestyle.
If we look at the study about them, we see that the average American man measures in at about 5’9″ (176 cm), whereas the average Dutch man stands at well over 6′ (185 cm) tall.