I had a look at those pages soon after posting. The gist is that the FT817 SWR meter doesn’t read SWR but reflected power. So at lower output power settings you will always get fewer SWR bars than at higher output powers. Also, the calibration is way off so it can only be a relative guide.
That’s designed as a dual band antenna (there’s a loading coil in the middle) whereas I was trying a 2m antenna on 70cm.