It could be that only half the force is initially applied because friction is immediately taken into account when you apply force.
And I’m not positive if that’s wrong or not. In theory i suppose it isn’t.
You could try lifting the cube off the floor before you apply the impulse to see.
To do it, enable simulate physics and immediately apply the impulse.
If the cube is 1mm off the floor, that’s enough to assume the friction came into play after the push.