|MadSci Network: Earth Sciences|
The short answer is that it isn't. By only looking at sunrise times, you are making an assumption that the length of a day is always (noon - sunrise time) * 2, but that isn't true. To figure the length of the day you need to use (sunset time - sunrise time). For mid-northern latitudes, the day of earliest sunset is around Dec. 7th. The latest sunrise is around Jan. 4th. And the shortest day is Dec. 21st.
Of course, this begs the question "Why do the earliest sunset, latest sunrise, and shortest day of the year occur on different dates?". For an excellent answer to that question, I refer you to the following website by John Holtz http://members.aol.com/jwholtz/analemma/analemma.htm
Try the links in the MadSci Library for more information on Earth Sciences.