The digital caliper reminds me that when I measure a post from my comprehensive assortment of sizes, that the stated size is subject to clearance tolerances.
Most posts are nearly 0.1mm smaller than indicated, and most somewhat more or less than that.
I really lucked out this weekend when I needed a 27.4mm post, but found an old Kingsbury/American Classic post in the 27.2mm-stamped size that was actually 27.27mm diameter. It fit really well in the 27.4mm seat tube.
All the other 27.2mm posts I had (10 or so of them) were significantly under 27.2mm.

Note that it's sometimes hard to ascertain the exact post diameter required using the inside-measuring tips of a dial caliper. You usually have to measure many positions and take an average, so it's better just to test fit the biggest post that fits without force.
A slipping post often suggests the need for a different brand of post that's just slightly bigger in actual diameter.

Those $10 calipers are awesome, and I've used mine probably 10,000 times over the last ten years. They cost $20 back then!