Force (Thrust) = mass times acceleration.
As the mass is constant, if the acceleration is reducing, then the thrust from the thrust reversers must also be reducing at lower speed.
-> speculation : as the aircraft's airspeed reduces, the ram effect reduces, reducing thrust. Slowing below 80-odd knots, the thrust reverser plume actually runs forward of the intake, starving the engine of air, and significantly reducing the engine's thrust output. Acceleration (decceleration) reduces accordingly.