I think those findings stem more from a misinterpretation of the data than actual scientific findings. People who are more susceptible to use cannabis recreationally are also more susceptible to not succeed in school. Think about it, if you're willing to break the law (in NZ where the study was done the repercussions are pretty intense) you're probably not extremely interested in doing well in school. You don't have to do much research to figure that out, just look around your local highschools. Stoners don't generally do as well as sober students in school. Since IQ tests reflect majorly academic standards, students who absorbed the information more due to proper study habits etc. would naturally score higher. Students that can't remember their algebra 2 because they were getting high in the parking lot instead of going to class wouldn't answer the questions as well.
