According to <
Wikipedia> ('cause I didn't really know the answer
, the problem is that the
definition uses the center of the Sun. Since the Sun is actually a rather large
disk, part of it becomes visible and starts lighting a spot on the Earth
before the center can be seen. Likewise, while it is 'setting' its disk is still visible even
after the center reaches the horizon. Therefore, the Sun's light makes the 'day' longer than the 'night'.
Then, there is the refraction of the Sun's light caused by the Earth's atmosphere. This extends the light even longer by a few seconds.
Thanks for asking, I never thought much about these facts. Even when flying into the rising Sun at O-Dark-Thirty (Hardly ever a pleasant experience!).
This whole problem could be fixed by moving the Earth a few hundred lightyears away from the Sun. Where's that <
Congressman> who was concerned about Guam capsizing when you need him?