Because they’ve spent 3.
The Ace spends 1 Tension to attack the other one and rolls a 6 again, taking out both.
One Tension spent on attacking the damaged ones.
The Ace then spends the rest of their Tension to make Assault Rifle attacks towards the Legionnaires at the top right, succeeding without consequence on one (5) and taking it out. They roll a 4 on the other, which is a success with consequence: that Legionnare is activated to attack, dealing 2 Harm to the Ace, but is taken out.
And two tension spent on two Assault Rifle attacks, one of which is a 5, one of which is a 4.