How are capacitor voltage ratings chosen
Real capacitors are far from ideal and different things affect their capacitance. For example, there are multiple types of ceramic capacitors that use different dielectrics and they all behave differently under temperature and DC-bias.

Some ceramic capacitors like Y5V can vary as much as -82% to +22% over their temperature rating! I'm not sure where these are used. I assume garbage consumer electronics. They come in very large capacitances for their physical size though...and they better with that -82% fluctuation.

X5R and X7R capacitors are better in that they vary about +/-15% over their temperature rating. These are typically used for decoupling for anything that matters.

But every type of ceramic except for C0G suffers from DC-bias effects. So if you apply a DC voltage to it (as decoupling capacitors are used) their capacitance decreases. Sometimes by 50% as you get near their voltage limit.

Don't use C0G for decoupling though. They are expensive and physically large for the capacitance they provide. They are also very low ESR (resistance) which is normally good but in decoupling applications can produce ringing. C0G is voltage bias and temperature stable and suffers from no piezo effects (other ceramics will produce a voltage under vibration which results in noise) so they are used in timing and signal path applications like filtering.

You can compensate for this by picking a capacitor rated at the size you want but pick a higher voltage than what you are running it at. Or you can pick one near the voltage you're running it at but choose one with a higher rated capacitance.

There are also tantalum capacitors (not ceramic capacitors) that are very sensitive to over-voltage and can also suffer reduced lifetime so you use them at half their voltage rating so bad things don't happen.

For electrolytics de-rating their voltage can extend their lifetime (also true of other capacitor types).