A block of mass \(m\) is attached to a light cord wrapped around the outer rim of a uniform disk pulley with radius \(R\) and rotational inertia \(I\), which is mounted on a frictionless horizontal axle. When the block is released from rest, it accelerates downward as the cord unwinds without slipping. A force sensor attached to the cord measures a tension \(T\) that is strictly less than the gravitational force \(mg\) acting on the block. Which of the following explanations correctly accounts for why the measured tension is less than \(mg\)?