3-Star Veigar without Items oneshot 3-Star Mundo

Can someone explain me why a 3-Star Veigar (without items and no shadow trait, but 3 magic trade) can oneshot a 3-Star Mundo (with Warmog's Armor, Dragon's Claw and Iceborne Gauntlet) ? Mundo had over 2800 hp while Veigar on max Lvl. can deal 900 Dmg with his ult so even if the magic trait is active it should not deal more than 1800 Dmg to Mundo (watched for double Veigar ult but Mundo was everytime only hitted by one Veigar ult). But my Mundo instantly died because the Veigar passive just triggered on him.