The statistics make perfect sense in my opinion. Remember Marijuana was made illegal in the early 1900's and since then methods of production, distribution, etc. have obviouly been modernized and expedited. Plus, it's not more than ever accepted by society (still not very much, but through pop culture it isn't AS bad.) It's easier to get than ever so a gain of 4000% in the last 100 or so years isn't too far fetched.
Plus, when something is legalized it loses that "cool" effect.