the paragraph goes on to explain that the difference is likely because the cyclists who actually bother to wear helmets in the netherlands are engaged in cycling activities (racing, mountain biking) that have an order of magnitude more danger than piddling along at 8 mph in a protected bike lane on a 60 pound omafiets.

while i certainly don't "race" on my daily bike commutes, with 8 miles to go each way, i don't exactly take it easy either, typically rolling around 17-18 mph (well for me anyway, i'm sure 18 mph is considered taking it pretty damn easy for a serious racer).

i would suspect that the faster one rides, the greater the potential benefits of a helmet are (ie. higher speed = higher likelihood of a crash).

but i have absolutely no data to back that up.