@About job advancement
You have the regular job advancement, and then you have the master class, which is NOT related to job, but to skill mastery.
To claim successfully your master class, sure, you have to level your advance skill to 100% (like fabio was trying to do). BUT, also, you need to fulfill the quest path (12 to 20 linked quest).
To rephrase it, the master class means you are at the peak of your skill. The only thing you can expect after that, is to claim and collect the missing special skill hidden within the continent.
In the case of Weed, he can learn other craft related skill and raised them up to mastery level. But that does NOT mean that he is all of those class; just that he was capable of learning a limited version of it.
For more informationhttp://the-legendary-moonlight-sculptor.wikia.com/wiki/Master_Quest
(this section need to be improved, comment welcome beneath the page)