Hi Guys

I have been experimenting with ISO and shutter speed. I took a series of photos outside which had direct sunlight in them as well as shade. I wanted to keep my aperture value the same so I took them in AV mode and changed only the ISO, increasing from 100 up to 6400. The results were what I thought would happen with the shutter speed increasing as my ISO increased... That made sense to me, the sensor is more sensitive so the shutter doesn't have to stay open for as long.
So then I took a series of photos inside, which was poorly lit and done the exact same thing of increasing my ISO. The results were not what I expected...the shutter speed got slower as the ISO got higher?
Can someone please explain why this is? Obviously it is not as straight forward as I thought