Why doesn’t the spring equinox have exactly twelve hours of daylight?

It would have exactly twelve hours of daylight if this were an airless world with no atmosphere.

However, Earth’s atmosphere acts like a lens and bends, or refracts, the Sun’s light above the horizon.

When you see the disk of the Sun resting on the horizon at sunset, you are looking not at the Sun but at a refracted image of the Sun, which is already below the horizon.

The image lingers about three and a half minutes after the actual sunset. For the same reason, the Sun’s image appears a few minutes before sunrise.

On average, the atmosphere’s lensing effect adds about six or seven minutes to the duration of daylight.

Day and night again appear to be equal a few days after the autumnal equinox.