Internal oscillators, even having calibration ( which I'm unaware if this uC has this feature ) they inherently have a very high variance when compared to the ones made with crystals, since they are based on RC networks implemented in silicon, and thus change their characteristics with the temperature. If you have some way to permanently read precise externals signals ( e.g 50/60Hz from mains ) you can atleast implement a correction factor to adjust the clock counter.