As stated above, the Earth's orbit isn't a perfectly symmetrical circle, but rather an ellipse. At the closest point to the sun, the Earth is travelling faster than at more distant points, and the Earth's angle around the sun changes more rapidly. This means that the Earth has to rotate farther to line a given point up with the Sun ("local noon") than when the Earth is farther away and traveling more slowly. As a result, the actual length of a solar day ("local noon") is usually somewhat shorter or longer than 24 hours. This means that in addition to the day time length increasing, its center point is also shifting, and as a result the sunrise and sunset times do not shift at the same rate most of the time. Solar noon varies by about a half hour throughout the year compared to the standard 24 hour clock.
Mike
ETA: This also means that although the Equator is generally considered to have 12 hours of daylight every day, this isn't (quite) true, and sunset and sunrise times will both change throughout the year. When the Earth is closer to the sun, the local day will be slightly longer than 24 hours and the night will also be longer. The difference is only a few seconds per day, however, so you wouldn't notice unless you were to compare it to a clock.
Mike