What are you trying to accomplish Mark? If you set it to off, do you want it to be able to shoot at any of the ISO settings (choose in auto) up to 1600--or do you want to be able to set your own ISO. IF so--I would just take it off auto and set to 100ISO and then just choose ISO as you want.
If you, for instance (as I do), perhaps not want it to go above ISO800 when its setting the auto ISO setting, then choose 800 as your max--or if you want it to choose any ISO up to and including ISO1600, then you need to set your max ISO to 1600.
I just tried it with auto set to off and it does indeed choose up to ISO400--not sure why that, so not sure why one would set it to off. But then--if you don't want to use auto ISO--the better way is to choose ISO 100 and then each shot choose your own ISO.
I suspect that what you want is to be able to shoot with auto ISO, allowing the camera to choose according to brightness, up to and including 1600. So--rather than set it to off--choose 1600 as your max number, click on okay and choose auto as your ISO choice.
Hope this helps.
Diane