Hi Polyglot: yes the focus only had to be adjusted slightly for the hyperfocal distance (vs infinity). The weak point in the test was the lack of something close in the very center of the frame, although the detail evaluated was not too far from the center. Field curvature was one of my guesses too.
Hi wiltw: This is what I always do, sometimes more than one stop too. I use CoCs quite a bit smaller than the usual 0.03 the manufacturers use. And one of the main reasons for this kind of testing was for me to find the proper balance between depth of field and CoC because while stopping further down is always good for depth of field, eventually diffraction makes things more fuzzy than if they were slightly less in focus. The issue you raise could explain why people get fuzzier infinity detail than they expect when they use hyperfocal focusing, but it still doesn't explain what I'm seeing, which is that the near objects are sharper when the lens is focused at infinity vs the hyperfocal distance.