That's because any attack (evocation, fist, gun, whatever) deals the difference between the attack roll and the defence roll as stress. Weapon rating is added on top of that.
So a power 6 evocation attack is thus Weapon:6
The caster rolls a total of 7 to control and aim (they're one roll). The target rolls a total of 4 to defend. The difference is 3 (7-4=3)
The target takes 3+6=9 stress. 3 from the attack roll itself, and 6 from the weapon rating.