if your measurements are accurate, a 28mm seat tube (Metric tubing) with PROPERLY sized 26.8 seat post means a wall thickness of 0.4, which is very thin-walled. This is in the neighborhood of Reynolds 753 tubing, lighter stuff than 531DB or Vitus 172 DB, AFAIK. Typical seat post for Metric 531 DB should be 26.4 (or sometimes 26.6) and note there's no butting in the top of the seat tube.
So I am still in doubt of something being correct in this here scenario: either the OD of the seat tube, the size of the seat post, or that the post fits that tube.
But I will stand to be corrected...