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?