Today is the spring equinox, which is one of two times in the year when the Sun is exactly above the Equator and day and night are of equal length all over the Earth. (The word equinox is Latin for equal night.) But if you look at the times of sunrise and sunset today, you will see that we actually have 12 hours and nine minutes of daylight. You have to go back to March 16 or 17 to get closest to exactly 12 hours.
It is the same for the fall equinox, which this year officially falls on September 23 but September 25 is when we have equal lengths of day and night.
Why is this? This article explains.
The Equinox is when the sun is in line with the Equator. The reason we do not see equal dark and daylight on the equinox has to do with how we view the sun. Sunrise is defined as when we first see the upper outer ring of the sun, of course sunset would be the exact opposite.
Atmospheric gases play a large role of when we see these outer rings of the sun to determine sunrise and sunset. The waves of light from the sun pass through atmospheric gas causing them to bend, reflect and refract. These changes to the sunlight can lead to a few minutes of change when sunrise and sunsets are observed.