I've got two sets of cheap digitals and about a half dozen dial calipers. I've checked them all at some time or other against the 1 inch standard that came with my Mitutoyo 1-2 inch mic. All the digitals read exactly 1.000 and the dials read either spot on or at most a half a needle's width from true.... about 2 to 3 tenths being the guess for the worst of them. And since yours both read the same as close as you can tell I'd guess that it's the posts.
While you're at it measure around the diameter at multiple spots and at different places along the length. I'll bet they are anything BUT round and likely have fat and skinny spots. Likely the only posts you'll find that are truly accurate are the mid to higher end ones that are machined or that have better quality drawn tubing.