It's VERY simple. If a 27.2mm post fit at all, it's the right size and a 27.0 will be too small. It is physically impossible to insert a post which is 0.2mm oversize.
The tightness may be because of ovalizing at the seat clamp area, or simply a tolerance issue. FWIW- the tolerances for seatposts are that the tube should be the nominal plus 0.00-0.05mm, and the post should be the nominal minus 0.00-0.05mm. This kind of tolerance is designed around reasonably achievable tolerances, ensuring that the post will fit without jamming or being sloppy.
Think about this as a 2 lane road with a double line down the center. The nominal is the double yellow, and the fog lines are the extreme limits. As a practical matter makers try to stay centered in their "lanes" without crossing the centerline,or running off onto the shoulder.
In your case, it's a tight fit because one or both are crowding the double yellow.
If you are uncomfortable with the fit, you can consider reaming the frame or trying another post, but do not ream the frame before confirming that the post is on spec. I see too many instances of oversize seat tubes, which is a far more expensive issue than buying another seatpost.
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