Mobs do not drop rare items, for some reason. This includes wither skeletons not being able to drop stone swords and wither skeleton skulls, but also rare drops of other mobs. To recreate, give yourself a Looting 10 sword, a high Strength level, and spawn in some wither skeletons. No matter how many you spawn, you will not be able to get skulls or swords from them at all if you are in creative mode. Once you switch to survival, however, everything returns to normal.