Normally on a camera; point or shoot or DSLR, using the Auto ISO is a bit of a risk as it has quite a high range and noise might become an issue to your image quality.
The Auto ISO on the G9 limits the ISO to a max of ISO 200 so the image quality is kept, even when cranking the aperture down to f8.0 in Av mode and it still stops at ISO 200 and if light is in the plenty, the Auto ISO will drop all the way back down to ISO 80.
So in short if you want to keep the images at high quality but are in changing light conditions switch it to Auto ISO 
