Yes sir. This is due to following of some orthodox christian sects following different versions of Julian Calendar. This kind of shifting of festivals we can see in India also. I have seen Ram Navimi is being celebrated in South Indian states on different dates.
And the slide show given in your link is simply superb.
Friends Christmas is one of the best festivals which is celebrated all over the world with lots of joy and fun. Dark winter Night becomes enlightened and colourful. I love it.
