I wonder if that’s because it’s close to the Solstice.
It has more to do with the earth being at perihelion on January 2nd. than the winter solstice. As the earth is closest to the sun in January, its speed increases. Hence, in early November, Solar Time was 16 minutes ahead of standard time, and by mid February, it will be 16 minutes behind. Christmas Day falls right in the middle of the progression. Check out this link. http://www.sundials.org/faq/
Sorry, if I’ve taken this post off topic,but I honestly believe the Church knew something about this astronomic fact when determining the date of Christmas.