When you do a typical bench press rep, it’s a little known fact that a good percentage of the last part of the movement (as you come to lockout) is used by the body to decelerate the bar. The lighter the weight you use, the greater this percentage is. The heavier you go, the lower this percentage is.
This is a safety mechanism used by the body to prevent joint injury. It senses the load on the muscles and joints and will actively work against the movement in order to slow down the resistance and protect the joints.