The behaviour of the Auto ISO algorithm is consistent when using the camera pop-up flash in slow mode as well. Unfortunately, things are weirder when using a hot-shoe mounted flash, such as an SB-400. In this case, the camera increases the ISO sensitivity up to four times the value currently selected and won't raise it any more, even if you selected a greater maximum ISO.
#1. "RE: Auto ISO with a Hot-Shoe Mounted Flash" In response to Reply # 0
Hatboro, Pa, US
The bottom line is don't use Auto ISO with flash of any kind with the D5100 or D3200 for that matter.
The camera Auto ISO will raise the ISO value to match the darkened ambient light in the room. In the case of setting the high ISO point to 3200 or 6400 with the built in flash it could raise the ISO up to that point.
I just set my D5100 to Auto ISO with a high point of ISO 3200. I then took two identical photos. With the built in flash ISO jumped to 3200. I had the default ISO set at 200. With the SB400 flash mounted, Auto ISO set the ISO to 800. So the bloggers 4X number seems correct with the SB400.