Stated in the article-
...However, it is difficult - perhaps impossible - to provide a nicotine strength or liquid volume equivalency between cigarettes and electronic cigarettes, because the nicotine is absorbed less efficiently from e-cigarette vapor than from cigarette smoke, and because there appears to be a variance between the tolerance to nicotine in cigarette smoke and that in vapor (as there is no correlation between the amount people smoked and the
e-liquid strength they end up using).