1. Coordinates never get stored in English units, only in meters, so conversions happen.
2. The Z resolution you have chosen allows this level of movement to be considered the same as an exact value.
3. Z Resolution establishes a Z grid and the Z extent sets an upper and lower bound of possible coordinates, and sometimes this makes it impossible to make whole number coordinates. Check and report these settings.
4. The projection chosen for Z may apply some adjustments when combined with the chosen XY projection (although that is a black box to me so I am not sure how they may interact).
5. Base 10 to base 2 conversions.
6. That distance is less than 1/10th the width of an average human hair and is such a small amount of measure variance from your reported benchmarks that it clearly falls within the tolerances of standard Survey procedures.