Cannabis use prevalence is actually kind of low in the Netherlands, many european countries where it's illegal have higher prevalences. Turns out, legalizing cannabis doesn't turn everyone into a stoner.
It must be on purpose right. Only thing i can think of is that there were enough studies done (opposed to in Belarus for example), but they contradicted each other.
The EMCDDA is an authority from the european commission, thus has gathered data from the EU (2014 including the UK) and the affiliated countries Norway and Turkey.