After playing awhile on my single player world, I've noticed that witches have not been spawning anywhere else other than these few certain chunks. Now because of their rarity, I needed to observe this over some period, but I can conclude that these witches are indeed spawning in these chunks only, nowhere else. My world was created before the 1.4 update, and after 1.7 came, these witches were only spawning in certain chunk locations. I have created a few superflat worlds, and they seem to be spawning like any other mob, in random chunks in random places. I do not know how to reproduce this bug other than to make a new world before the 1.7 update (maybe lower?) by changing the version in the launcher, and go back to 1.7 to see spawning behavior.