If you were just holding the RES/+ button until the desired speed is actually reached and letting off the button, you most likely did not notice the speed setpoint vs looking at your speedometer.
The owners manual explains it clearly
Pushing the RES (+) button, or SET (-) button once will result in a 1 mph speed adjustment. Each subsequent tap of the button results in an adjustment of 1 mph.
If the button is continually pushed, the set speed will continue to adjust in 5 mph increments until the button is released. The new set speed is reflected in the instrument cluster display.
It is a setpoint to a computer that you are programming with the increase/decrease buttons. In your display you can actually see the setpoint as mentioned above