Originally Posted by Moe Zhoost

Measuring a seat tube with calipers will give you inconsistent measurements. The insert type of gauge is best. I'd say that from your measurements, the 26.6 spec is probably accurate. It's clear that the cable hanger is interfering with the clamping, and this is easy to check. To fix, you can move it to the side as others have recommended, mount a different hanger (like the one in the picture below).

I would recommend first to file the hanger a bit thinner at the clamp end. Half a mm make make all the difference.