Atmega Internal clock source accuracy

Hi,

I find that Atmega internal 1MHz RC clock source is inaccurate. It is
about 95% of 1MHz. I used 3V.

I have tried to check the oscillator calibration byte by AVRStudio and
then load the value into OSCCAL in the beginning of the program.
However, the problem is still present.

Could anyone tell me how to calibrate under this condition?

.