If you go to the AVR site and open an AVR application note AVR053, you will notice 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 the oscillator introduces better features and improvements. But is it true? ChaN has done exciting research on this oscillator version. He tested the output signal with fixed width and measured timing fluctuations of it. And he found out that the 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 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.
You can see that chips introduced 2-3 years ago show almost no noticeable jitter in the picture, while new ones give about 0.5% maximum. It seems that significant fluctuation appeared from the 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 a more significant RC jitter. But this is only guessing as Application notes do not tell anything about this issue.
Probably intentional to help in EMC issues.
See:
http://www.eetimes.com/design/analog-design/4213529/Reducing-EMI-in-digital-systems-using-spread-spectrum-clock-generators–Part-1-of-2-a?pageNumber=0