Yes, the 1.1V have about 10% accuracy.
The spec sheet gives a nominal value of 1.1 volts, but states that it can vary from 1.0 to 1.2 volts. That means that any measurement of Vcc could be off by as much as 10%. Such a measurement could be less accurate than our power supply for the Arduino!
http://provideyourown.com/2012/secret-arduino-voltmeter-measure-battery-voltage/