When AUTO ISO is enabled, you get to set the max iso and the min shutter speed in that same sub-menu. So far, so good. But the regular ISO menu, the one you use to select the ISO when you're NOT using AUTO, also plays a role. Whatever its set to is the MINIMUM ISO the auto ISO will choose. So, my guess is you had the manual ISO set to 800 before you switched to AUTO, figuring that would override the manual setting... But NO, it only changes its role in life. The other quirk is that while this camera treats almost nothing as sticky, it treats the ISO you were last in for any given mode as sticky. So if you were shooting at 800 in aperture priority mode, switch over to shutter priority mode and lower the ISO to 200, when you go back to aperture priority, that sucka's going back up to 800.
Its a screwy system that MAY have some logic behind it, but if so, its too convoluted for my little brain to quite work out. So you just have to learn its ways and play by its rules...
As the Eagles once advised, "don't even TRY to understand, just take it easy". Seems to be the best advice for dealing with some of this camera's quirks.
-Ray