Remember that the Earth orbits the sun, not in a circle but an ellipse. The ellipse as shown in this diagram has been greatly exaggerated. While the winter solstice occurs on December 21st, the Earth does not pass closest to the sun until about 2 weeks later around January 3rd. The overall "figure-8" shape of the analemma is not affected, but the angle has the effect of slightly skewing the analemma shape at the top and bottom.