Framebuilders - seatpost size and reaming

I have a question. I'm not a framebuilder but I'm confused/curious about this.

How does one obtain a a given seat post size on a bike?

Ex. My road bike is Columbus SLX, with a 0.6-0.9 single butted seat tube (well it should). Since the tube OD is 28.6 the ID should be 28.6 - 2*0.6 = 27.4, yet it takes a 27.2 post. I have another bike which is Columbus SL with the same butt profile and takes a 27.0 post. Finally my touring bike is Columbus SP and takes a 27.2. This one makes sense since it's butted 0.7-1.0 (28.6-2*0.7=27.2) but I was told they where usually made for a 27.0.

So I'm wondering how it works since I'm obviously wrong. Is the tube actually double butted then reamed one or two sizes up?

Do the math and add 0.2mm for clearance.

Common 1-1/8" OD seat tubes with 0.6mm wall at top take a 27.2mm seat post. These tubes are single butted with the thick wall end down at the bottom bracket and the thin section at top - which is the end that is trimmed to size when building the frame. When brazing the seat cluster there will be heat distortion thus the need for reaming. Some people don't do a complete reaming job and wind up with a slightly undersized ID (this could be the case with your SL frame).

I've built frames using a True Temper double butted seat tube which have a funky .8mm section at top. Used my reamer and just skimmed that butted section down until it was 0.6mm which allowed a normal 27.2mm seat post to fit inside.:)

Ok I see it's because the steel contracts during brazing and stays contracted after. Makes sense! Thanks.

Just as a general priciple, you don't want to ream the tube up a size. Normally the seat tube would fit nicely in the tube before heat. There are probably situations where the jump is pretty minimal, but as has been said there are distortions that need to be dealt with, or may need to be. There are a number of ways of reducing the chance there will be distortion. Like adding an internal sleeve, or even an external one.