The calming effect of weed could correlate with better regulation of brain function. Reduced distractions. Both of which correlate with better decisions and higher intelligence.
Its hard to say what effects marijuana has. Most major studies used weed bought off the streets. Samples which could have been tainted with pesticides and other toxins.
I think its generally agreed multi tasking and distractions reduce intelligence and performance. The way a person driving their car and talking on their cell phone would expect driving IQ to fall. Weed trends towards an opposite effect. It helps eliminate distractions. Cuts down on doubts, anxiety and worry. Which could correlate with higher IQ.
Intelligence correlates with knowledge. Weed won't expand a person's vocabulary or knowledge base. It won't make the brain bigger. It can only build on the existing information people have and remember.