Put simply it's a function of the (mis)alignment of earths orbit and the tilt of its axis of rotation.
The middle of the daylight period isn't always at noon GMT, and in early January it moves later fast enough that dawn actually gets later in GMT for a few weeks.
It seems for a while as if the mornings are still getting shorter while the evenings are getting longer quicker than they should (suits me). It all corrects itself later.
That's why the accuracy of sundials relative to GMT varies during the year. If we used strict solar time this wouldn't happen, but clock design would be a bit of a nightmare!
I saw a good animation of this on a visit to London's Science Museum once, but I don't think it's a part of their permanent display. The model explained what happens beter than words ever could.
