It's nothing as elaborate as that.
It is simply that the Christmas message was painted on the window from the INSIDE.
It's done with some sort of white-wash which, when it gets wet from the rain, would wash off.
So it's painted on the inside so it lasts.
But when you're painting it on the inside, you have write BACKWARDS.
And a lot of people can't do a backwards S, the brain doesn't seem to allow it.
So the S ends up being wrong.
Whether the design team deliberately did it like this, for comedic value, I don't know.
But somehow it looks more natural being wrong.