I've measured the tubes again and I got 26mm for the top tube. The seat tube seems to be slightly elliptical, wider by 1mm in width measured across the bicycle than when measured along the length of the bike, making it 28mm one way and 29mm the other. The down tube is still resolutely 29mm. I also think the calipers are accurate, I measured one Spanish Zeus I have and got exactly 26/28/28 and a couple of seatposts with diameters stamped on them and got accurate measurements.