>So if a screw spins with 5 in-lbs of torque, then you simply can't apply 7 in-lbs - it moves before 7 is reached.

Or if you did apply 7-in pounds, you would accelerate it rapidly and it would spend very fast, which isn't really possible with a manual tool although it could be possible with an electric tool, but spinning very fast would make it hard to avoid overshooting the final level of tightening that you want to achieve, and the whole thing becomes simpler when you go slow enough that inertial forces are negligible.