After some research and studying the graph you posted with your question, there are many reasons why all four latitudes have the same number of hours of daylight on March 20th and also September 22nd.
Some of the reasons are:
These two dates are equinoxes, the sun's direct rays are over the equator, and the Earth's North Pole is not is not tilted toward or away from the Sun on those specific dates. Any of these answers are acceptable for the question you asked.
