that could be dependent on the software at hand.
I've experienced similar effects when using reason, and it emulates analogue circuitry.
That being said, and with a little bit of digging into the analogue world, oscillators weren't always in pitch 100 percent of the time. they get out of tune over time.
I don't know if serum has certain wavetables that emulate that or if it's the synthesizer itself.
I don't think you should be to concerned with it, and if you do, bounce to audio because that's a surefire way to keep the sound exactly like it is.
or there's something wrong with your synth....not sure, it could go either way.