8 MHz internal oscillator (HFINTOSC) are factory calibrated. These calibration values are stored in fuses located in the Calibration Word (2009h). The Calibration Word is not erased when using the specified bulk erase sequence
ну и в описании регистра OSCTUNE написано: 01111 = Maximum frequency . . 00000 = Oscillator module is running at the calibrated frequency . . 10000 = Minimum frequency