Why the disparity in day length associated with the equinox?

At 6:44 am CDT on Friday, March 20, the overhead (vertical) sun passes over the equator (0 degrees latitude) on its seasonal migration into the northern hemisphere. This is the

vernal equinox

for the northern hemisphere (autumnal equinox for the southern hemisphere). With the sun vertically over the equator it is assumed that the length of daylight is roughly 12 hours everywhere, though there are variations with latitude that amount to several minutes. What goes into producing these variations? One explanation is time zone geography, and whether your location is on the eastern side of a time zone or the western side of a time zone. In addition if you checked the sunrise and sunset times in your local newspaper or from a web page, you would probably find that as of last Tuesday (March 17th), the sun should be above the horizon for at least 12 hours at most locations, and by Friday, March 20th it is above the horizon for more than 12 hours at many locations. The effects of atmospheric refraction (bending of light rays by the varying density of the atmosphere) along with a relatively large diameter of the sun (disk size), and slight variations in the angle of the setting and rising sun relative to the curvature of the Earth all contribute to several additional minutes that the sun appears above the horizon at sunrise and sunset. This of course varies with latitude, even in Minnesota. We will gain an additional 34 to 36 minutes of day length over the balance of March and continue to increase day length until the summer solstice on June 21st.