Just noticed, second game in a row, that if the opponent has the 5 armour from the tavern spell, it counts as 10 in the damage calculation.
The opponent had 11 health and 5 armour
I hit him for 10 dmg, and they was left with 11 health 0 armour…
The other game they had 3 health 5 armour
Hit them with 11 dmg and they were left on 2 health
Edit 1: It seems it calculates properly when the damage is fatal