Range of seat post diameters depends on the OD of the seat tube , the wall thickness of the Tube ,
and whether the finished frames seat tube was reamed out when finished ,
a hand reamer is an adjustable cutting tool with straight sides..

so I for example , have enlarged my frame's ID to suit, have an RB1.. they ship as a 27.0,
so the fitting of a 27.2mm post required shaving out 0.2 mm from the upper portion of the seat tube..

Many light steel frames have a 27.2 ID when finished. abundant product choices in that size.

had a standard Diameter tube Aluminum AlAn, it used a 25.0 seat post..

some companys, like USE make just a few sizes of seat post, then Shims fill in the sizes in between.

a perfectly valid method .