Yes, this question is often brought up about Narnia because it seems like a clash of worlds. I think it's a mixture of these three following ideas, though I also think you could pick just one and stick with it if you wanted.
Explanation #1. It's a children's book, and Lewis wanted to make winter sound as horrible as possible, so he tried to take out all the things children see as good about winter. Christmas was one of those things, so even though it was an anomaly, he pointed it out.
Explanation #2: In our world, Christmas represents the beginning of the hope/change that Jesus will bring. In Narnia, Christmas represents the beginning of the hope/change that Aslan will bring. It's not really there to mark an
event, since we know that it also happened before the White Witch was in power. Rather, it marks an
idea-- of joy in the midst of a dark and cold world.
Explanation #3: Lewis loved myths and mythical figures. Into Narnia he places dryads/naiads, fauns, Bacchus, and many other figures from stories in our world. Even among these figures, there are a lot of incongruities. Sometimes fauns have short tails, sometimes long. Sometimes dryads seem like actual trees, sometimes like spirits of trees, and sometimes like people who live in/are connected to/are the shepherds of trees. Father Christmas is another such seemingly incongruous mythical figure. Maybe he just threw in what ever seemed to best fit the scene or moment or idea he was thinking of for the piece. Or, maybe he had some deeper ideas for all his incongruities. Narnia is a 'supposal'-- a 'what if'. Lewis asks the question "What if" God were a Lion in a world filled with talking beasts. He seems to think the answer is, "Well, in such a world, all these fantastic figures would be real as well, including Father Christmas". In such a supposal, the wider ideas are the most important, and the details (might be) less important.
This article has some interesting thoughts on Lewis' incongruities:
http://www.cbn.com/spirituallife/inspir ... tion=printWe have some real Lewis/Narnia fans here who may have other theories as well.