If you go to AVR site and open an AVR application note AVR053 you will notice that there are different RC oscillators installed in to AVR chips during history. Simply speaking each new version of oscillator introduces better features and improvements. But is it really truth? ChaN has made interesting research on these oscillator version. He simply tested output signal with fixed width and measured timing fluctuations of it. And he found out, that RC generator frequency slowly fluctuates during time.
AVR internal oscillator jitter research - [Link]