The authors factored in for people who smoked tobacco and those that lived in more polluted areas with lesser air quality. The harm from cigarettes showed up clearly while those smoking a joint a day and not smoking tobacco did not show the degradation. Even one joint per week for twenty years did not appear to have significant effect.
Its not known exactly why tobacco appears to be so much more harmful than marijuana, especially considering the contents of the smoke are similar. It is known that THC, one of the main active cannabis oils in the herb, has anti inflammatory properties that may help to soothe the lungs. A part of the increased capacity was put down to the way pot smokers usually take deep breaths when they smoke, but one joint per day is hardly giving your lungs great exercise.
Obviously more research is needed, and it would be interesting to see results of lung tests in communities such as Jamaica and the Himalayas where smoking pot is endemic and done in larger daily volumes.