Seat post nominal and actual size
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.