All Spirit treadmills are designed with a slight incline that more closely simulates walking/running outside on a flat surface. Even when your treadmill is set to the zero (0) incline setting, you will notice a slight manual incline. This slight incline also helps prolong motor life since it has less strain on the motor.
Depending on your treadmill model, the incline settings may vary. To learn more about your treadmill's incline settings, review your machine's Owner's Manual.