In my sampling (one bike) the 26.2 mm seat post diameter is required because a sleeve is brazed in as a liner for the top 3 cm of so of the seat tube. This has the nice effect of greatly reducing the odds of the seat post freezing in the tube; just much less contact area. Unfortunately, my inside guage doesn't know how to measure the greater diameter below the lesser diameter, so I can't tell you the "true" id of the seat tube.
harvey sachs
mcLean va