Originally Posted by T-Mar

A 0.3mm increase in diameter would be very thick for paint. Typically, the increase in diameter would be ~0.1mm. Since the increase is consistent across all three frames, this suggests your calipers need calibration or possibly there is some foreign material on the jaws that requires removing.

You're making an incorrect assumption in your calculations. The SL and Cromor seat tubes are single butted. That is why the seat tube specs say 0.9/0.6 rather than 0.9/0.6/0.9 The 0.9mm butt is at the bottom of the seat tube to stiffen the bottom bracket region, where stresses are higher. The top, 0.6mm, non-butted end should be used to calculate seat post size. You should also deduct a further 0.2mm from the calculated inner diameter of the seat tube to allow for clearance fit.

Imperial seat tube inner diameter for SL or Cromor = 28.6 - (2 x 0.6mm) = 27.4mm
Correct seat post size is 27.2mm

Metric seat tube inner diameter for SL or Cromor = 28.0 - (2 x 0.6) = 26.8mm
Correct seat post size if 26.6mm

Your seat posts are all 0.4mm (2 sizes) under the correct size. I assume the reported seat post sizes are those stamped on the posts because if they're measurements, the actual post sizes are almost certainly smaller based on the errors in your seat tube diameter measurements. I know it doesn't sound like much but that a significant difference when talking tubesets. Again, it's curious that the difference is consistent across all three frames. I wouldn't expect that if it was due to something like tube distortion. Typically, these sizes seat posts would be used on tubesets with walls that are two steps thicker than SL or Cromor. The appropriate Columbus tubeset for those size seat posts would be Aelle.