From Android Sensor API:
public static final int TYPE_STEP_COUNTER
A sensor of this type returns the number of steps taken by the user since the last reboot while activated. The value is returned as a float (with the fractional part set to zero) and is reset to zero only on a system reboot.
(Emphasis mine)
As you can see, while the sensor is activated, the value will keep increasing without resetting to zero until the system is rebooted.
If you want to count from 0 every time the app is started, you can store the first value returned as initial value, then subtract subsequent value by it.