If you go to AVR site and open an AVR application note AVR053, you will notice that there are different RC oscillators installed into AVR chips during history. In the table, you can see tunable oscillator versions and their features.
Simply speaking each new version of oscillator introduces better features and improvements. But is it truth? ChaN has made exciting research on this 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 the time. Of course, RC oscillator fluctuations are not a problem as this type of clock source isn’t considered to be stable. In time critical applications it is better to use crystals. But the most exciting thing is that newer versions of tunable oscillators were generating much more jitter than older ones.
In the picture, you can see that chips introduced 2-3 years ago show almost no noticeable jitter, while new ones give about 0.5% maximum. It seems that significant fluctuation appeared from 4.0 RC oscillator version. It is hard to tell the reason why it is so. Probably it is a result of some efficiency improvements, that resulted in more significant RC jitter. But this is only guessing as Application notes do not tell anything about this issue.
You can read the full article here. It is Japanese, so use the Babelfish translator.