If the sun were just a tiny point of light and Earth had no atmosphere, then day and night would each be exactly 12 hours long on a spring equinox day.
But to begin with, as seen from Earth, the sun is nearly as large as a little fingertip held at arm’s length, or half a degree wide.
Sunrise is defined as the moment the top edge of the sun appears to peek over the horizon. Sunset is when the very last bit of the sun appears to dip below the horizon.
The spring equinox, however, occurs when the center of the sun crosses the Equator.
Plus, Earth’s atmosphere bends the sunlight when it’s close to the horizon, so the golden orb appears a little higher in the sky than it really is.
As a result, the sun appears to be above the horizon a few minutes earlier than it really is.
Therefore, on the spring equinox day, the daylight hours are actually longer than the length of time between when the sun crosses the horizon at dawn and when the sun crosses the horizon at sunset.