Seat post nominal and actual size If seatpost has nominal size of lets say 27.2, what is its actual size? Or other way around, if frame nominally accepts post of 27.2 what is the actual size of the bore in the frame?
Obviously both can not be 27.2 since it would not be possible to insert the seatpost, so what are actual dimensions?
Please check which one refers to internal diameter and which one to the outer.
If the seat tube (the one on the bike) has an inner diameter of 27.2 and the seat-post has an outer diameter of 27.2, obviously they will match.
You can expect tolerances (variances, as more correctly mentioned by @weiwen-ng ) on the order of +-0.2mm, unfortunately.
Don't forget grease, especially when installing aluminium seat-post in a steel seat tube.