Martine (sorry can't seem to tag you for some reason), that seems very strange to me. The assessment period is supposed to run until the 14th so if your guest stayed during December and left the review by the 13th it should definitely be taken into account. I would contact Airbnb to question this.
However, first, make sure you have your maths right. The figure shown on your stats is normally for the current period, not for the last one. Are you sure the 80% 5 stars you achieved relates to the period being assessed, e.g. that it doesn't also take into account any 5 star reviews for guests who stayed in January?
Also, it's not just the 80% you have to achieve, but also an overal 4.8 star rating, as well as certain levels for commitment, response rate, % of your guests that left a review etc. So, if you are confident you met all of the criteria, then I would contact Airbnb and ask them why you didn't get Superhost.