By internal oscillator you mean the RC internal oscillator, that has nothing to do with eternal Xtal oscillator.

The RC will always be off because of tolerances and temperatures.

Now, regarding the Xtal itself, will be off a bit by construction. But little and well defined on the datasheet. The most important factor though, for the precision of the frequency is the loading of the xtal (some are loaded in series, most are parallel loaded). User have to read the datasheet and see what capacitance needs the Xtal so it operates as expected. Too much or too little loading may result in slower or no oscillation. One of the loading capacitors may be a trimmer cap and be adjusted to obtain precise frequency.

Ioannis