On a recent trip we were struck by the fact that virtually every Dutch tourist industry service worker we encountered spoke idiomatic fluent "American" English, almost like they were native speakers. We were curious, because of course we've encountered many good European English-speakers, but in other countries they usually don't sound like they were born in the US.
So I asked one of these American-speaking locals why he spoke "American" so well. And this is what he said: Because The Netherlands and Belgium are relatively small TV markets, when he was a child, the Dutch TV networks didn't dub over with Dutch the American sound-track of American TV programs shown in The Netherlands. He said this resulted in the Dutch children focusing on the spoken American, especially those not yet old enough to read the Dutch subtitles (presumably the Flemish subtitles in northern Belgium or possibly French subtitles in Wallonia or Brussels).
Combine the above explanation with the scientific theories (this is what I've read, I claim no expertise in this subject) that the younger human brain is especially good at absorbing spoken language. In effect, you had a pervasive American language lab occurring in The Netherlands and Belgium.
Since I returned from Belgium/Netherlands a few weeks ago, I've done a little informal research on "why the Dutch and Belgians speak such good American." Naturally, there are other reasons for this, such as: residents of Belgium and Netherlands are given English classes in high school (but these may be in "British" English, along with German and French language training. Also as residents of small countries, the locals find it particularly useful to acquire English language skills for business or other reasons. Then let's not forget the regional linguistic and cultural division of the country we call Belgium between Flemish, Wallonia (French), German (apparently in a couple of areas of eastern Belgium); and bi-lingual Brussels.