We run two Atmega644 in our device.
One runs at 20MHz external crystal, it's an ATMEGA644 20AU.
The other runs at 8MHz internal oscillator, an ATMEGA644V 10AU.
The latter one ALWAYS does ~12 Âµs SCL stretching at the end on every byte received. The former one does not have this problem.
Any hint as to why? It is not much of a problem, more an annoyance.