The camera uses automatic exposure to achieve that - the exposure time that's configurable in that drop-down isn't the fixed exposure that will be used - it sets the slowest exposure that the automatic exposure will use. As the image gets brighter, the exposure time will reduce. As it gets darker, the exposure time will increase, but will not go larger than the value set in the dropdown.
*edit* And by the way - under 'Day/Night switch' presumably you want to change the setting from Auto to Day