Someone correct me if I'm wrong, but doesn't ios auto brightness work as follows:
Set auto brightness on.
Set slider to position to where you want the phone to be set as the MINIMUM darkness you want the phone to go.
Example. Go to a dark room. Set brightness on auto and slider to 50%. Flash bright light at light sensor. Watch screen quickly get bright. Remove bright light from the sensor and the screen brightness will slowly go down to 50% but not lower. Now, do the same thing, but this time start your brightness slider at 0% and then apply light source. Now when you remove the light source, the slider will slowly but eventually go all the way to 0%.
If I'm correct, the slider serves as a minimum setting on auto brightness.