My understanding is that English is a natural second language for the Dutch and is taught extensively in schools (i.e. learned by everyone). Also, they are close enough to the U.K. to receive broadcast television, etc. so they also have that level of immersion.

Every Dutch person I've met has had a terrific command of the language, much better than average native speakers. I suspect that at least some of this is due to the fact that as second-language students they actually take time to learn the rules of grammar, etc. Most native speakers pick it up "on the street", so the Dutch (that I've met) tend to sound more formal and educated, especially when there's no discernible accent.