If you check your sunrise and sunset times you will notice the length of daylight is not 12-hours on the equinox. The specific date for 12-hours of daylight varies with latitude.
There are two reasons why 12-hour day length does not occur on the equinox. The first deals with how sunrise and sunset are defined. Sunrise is defined as the moment light becomes visible from the sun's disk. Since the edge of the sun's disk is visible before the centroid (center point) of the sun, this produces a little more daylight time. Sunset is defined as the moment the sun's disk is no longer visible. Sunset occurs after the sun's centroid has set. This also produces a little more daylight time.
The second reason deals with refraction within the atmosphere. Refraction makes the sun's disk appear higher than it really is. This produces more daylight time than would occur if the earth had no atmosphere.
For people living in the mid-latitudes, the period of exactly 12-hours of light occurs a few days after the fall equinox and a few days before the spring equinox.