Generous application of the Blown Across the Room trope.
Bullets and shotgun pellets are streamlined, hard objects which focus a large amount of kinetic energy onto a small area, but has relatively little momentum due to their small size in comparison to a human, meaning it has little ability to drive an object back.
In the overhead shot of the residents walking out of the building, four soldiers are standing at attention. However, as Su-na walks out and down the stairs there are now only three. Then in the next cut of Su-na talking to the commander and female host there are four guards again.