Whenever a village naturally generates, all of the villagers in that village that would have been farmers will all have a specifically different profession for that village. This is noticeable when a village with lots of farmland has an overabundance of specifically Blacksmiths, Librarians, Clerics, or even "Nitwit" villagers. Currently, the only way to get a farmer villager is to find its respective zombie counterpart and cure it.