Inconsistent exposures at "in between" ISO settings on Canon 6D mkII

Over exposure at some ISO settings

I started out this evening trying to figure out how to compare noise levels at various ISO settings on my Canon 6D Mark II camera. But instead, I noticed a pattern of overexposure at "in-between" ISO settings. By "in-between" I am referring to the settings between the main 100, 200, 400, etc. These would be 125, 160, 250, and 320.

The exposures at ISO 100, 200, and 400 were consistent with the camera in AV (aperture priority), as were those at 125 and 250. However the photos at 160 and 320 were overexposed by about 1/3 of a stop (the pattern continued throughout the ISO range up to ISO 40000). Of course the first thing I checked was the shutter speed to see if the metering was somehow off in those two photos. No, the shutter speeds were correct at 1/20 and 1/40 (see the progression of shutter speeds below). And I was using continuous light, so it wasn't inconsistent flash power.

How about an inconsistent shutter speed? No. I changed the light a bit and tested again. Same pattern of overexposure, but this time the shutter speeds were the correct 1/25, 1/50, and 1/100. The overexposure is happening due to something else going on in the processing of the image in-camera.

1/10   1/13   1 /15   1/20   1/25   1/30   1/40   1/50   1/60   1/80   1/100

Here is the set of image where you can clearly see that the photos at ISO 160 and ISO 320 are overexposed compared to the other images.

I tested this on two other Canon cameras, a 5D Mark III and a M5 mirrorless, and did not see this issue with either of those. Exposures at all ISO settings were equal.

This morning I added in another set of images lit with flash and see similar results. I took my meter reading at ISO 100 (f/4) then raised the ISO in 1/3 steps while stopping down 1/3 stop for each image in the progression. Again, ISO 160 and 320 were overall a little bit brighter than the other images.

Have you done a comparison of exposure or noise at various ISO settings on your camera? If you have a 6D Mk II please let me know if you see the same exposure issues at the +2/3 ISO settings. 

Oh, here is a set of closer crops in case you want to see the noise comparisons...

At these low ISO settings I cannot see much, if any practical difference. But I will stick with ISO 100 for most of my studio work and bump it up a bit if I need a little extra aperture for more depth of field.