Recall that in a collision, the physical contact seizes to exist (if the carts do not stick together), and so the force felt by the force sensors decreases as the intensity of the contact between the carts decreases, eventually until the force felt after the collision by the force sensors is zero. This is not the case when there is a constant acceleration. As the carts are stuck together and pulled with a constant acceleration the force felt by the force sensors will always be present as long as the constant acceleration is present.