You probably redirect based on the first found locale. Most Dutch users browser with an accept language similar to "nl,en-GB;q=0.8,en;q=0.6,en-US;q=0.4".

If you only match on the first ("full") locale and not paying attention to the lower q-valued locales, you might think English is not accepted and you will redirect to your (default?) German site. It's a pretty common bug in locale detection systems (as I designed a few ones myself) :)