Yes, royal christmas card photographs are hardly ever christmas themed - not in the past nor nowadays. Who is surprised or criticises this year´s "happy-christmas-card" from the Cambridges is obviously not familiar with royal christmas cards!
Royals very often chose important events from the almost bygone year as images; from the old days I can recall wedding photographs of King Constantine and Queen Anne-Marie, the new british monarch chose a family group with her husband and 2 small children taken in the throne room after the 1953 coronation, or her mother another coronation-Cecil Beaton-portrait in coronation robes together with her younger daughter.
When Prs. William and Harry were younger there were these gorgeous black and white photos together with their father, often taken at Highgrove or in the Highlands of Scotland, no christmas themed what so ever, etc etc. Or remember this year´s card of the P o Wales and D o Cornwall in Cuba!
Perhaps some people might think photos from christmas photo shoots for the media, like the norwegian royal family does about a week before x-mas, are being used? Well, it is not - these photo-shoots have nothing to do with royal christmas-greeting-cards.