Heir mal nen Auszug aus dem Datenblatt:

During Reset, the 1 MHz calibration value
which is located in the signature row High byte (address 0x00) is automatically loaded
into the OSCCAL Register. If the internal RC is used at other frequencies, the calibration
values must be loaded manually. This can be done by first reading the signature row by
a programmer, and then store the calibration values in the Flash or EEPROM. Then the
value can be read by software and loaded into the OSCCAL Register.
Ich denke das dürfte alle Klarheiten beseitigen

Grüße,
Hanni