Ferber does say that normal amounts of sleep can range up to an hour (but rarely more than that) on either end of the ranges he lists. So if he lists 11-12 hours total for a 2yo, then the range really could be 10-13 hours. Depending on the child. I think he also says that children who appear to sleep extremely long may not actually be asleep that whole time - they may simply be happy to lie there awake or play quietly while the parents get their sleep.
All of which just tells me 1) our girls do NOT lie quietly awake or play quietly if they are not sleeping and 2) we're always in the Ferber range. Blah. Really I should move this to the BP We've NEVER had them sleep more than 11 hours at night, even at the age at which they needed the most nighttime sleep. And the 11 hours/night only lasted several weeks, during which we got excited and got a Netflix membership. Now we're wasting $10/month on Netflix