From the factory Atmel guarantees the internal oscillator to be ±10%. Which is a significant error. In my experience the error is always much less from the factory. More like ±3%.
They also guarantee that the internal oscillator can be tuned to ±1% by adjusting the OSCCAL register. In my experience, when the temperature and supply voltage are stable, the internal oscillator can be tuned to ±0.1%.
But, the oscillator is quite stable (some jitter; consistent average) when the temperature and supply voltage are stable meaning that the error is constant. As @JCA34F mentioned, if the oscillator is stable in your application then adjusting the delta (or adjusting the millis values) is a great choice.
In other words, yes, the internal clock could easily be running a little fast.