It's not only the Spring Equinox but also the Autumnal Equinox where that happens, and it's because of the motion of the Sun across the equator.
What your missing is that different points in the day, the angle subtended between the ecliptic and the celestial equator changes - because the Earth doesn't rotate inline with the ecliptic but an angle of 23.5 degrees - and this is most noticeable towards the solstices.
Since none of us are managing to convince you in terms you can understand, download a copy of Stellarium
http://stellarium.org and see for yourself by advancing minute by minute or hour by hour until you are satisfied that is true...
