if he has 13 armor, shouldn't the defense base value be '13'
Hero base armor (i.e. without bonuses from items or abilities) is calculated like this:
Defense base value (from Gameplay Constants) + Agility * Armor per agility point (from Gameplay Constants) + Defense base (from Object Editor)
By default each hero starts with -2 Armor (from Gameplay Constants) and gains 0.3 Armor for every Agility point. Thrall starts with 18 Agility at level 1 and 0 defense base in the Object Editor, so that's 18 * 0.3 - 2 + 0 = 3,4 base armor at level 1. Which matches what you should see in game.
You can alter it by changing appropriate fields in the Object Editor or editing the Gameplay Constants:
More specifically, the highlighted field and the one above: