Some villagers have Career that their Profession doesn't allow (e.g. a Butcher with Career 3 or 4). This can be found using /entitydata.

These villagers have trades from other random professions, but according their Career (e.g. a Librarian with Career:3 can have Tool Smith's or Shepherd's trades). If Profession of the affected villager allows multiple Careers, then it's name is displayed as a "Villager".

Old description:
While exploring the new 1.11 snapshot. I was observing villagers and any changes including the "nitwit." While in a Butcher villager's GUI, instead of displaying "Butcher" it displayed "Villager." I found other "Butcher" villagers and they displayed the name properly. I attempted to replicate the glitch by spawning in more villagers, but I could not. The villager occurred in a naturally spawned village. The coordinates are -500, 70, -700. The seed is: -7989118705340891292