That little area of ice at the base of the stab is were there is a big metal spar right under the skin where the fin attached to the fuselage. This coldsoaks in the cruise but has enough mass that unlike the skin and other lighter components in the tail structure it does not always warm above freezing up during the descent and so frost condenses on it on the ground.
You'll sometimes see similar effects elsewhere on the airframe (usually on the wing) where heavier structure lies close under the skin.