I'm using 2.8 and If I have an object that is say 10.75 on the z axis the read out will change to 10.8 once I've hit enter. It doesn't change the dimension to 10.8 and if I click on the box again it shows me that it's 10.75. It mostly just annoys me and makes me constantly click the box just to double check that my dimensions are correct and I haven't made a mistake.
By default blender uses metric measurement system with Unit System set to None and rounding to 3 decimal places wich best suited for real world objects:
If you using Impereal Unit System rounding wich default to Feet rounding can be from 0 to 3 decimal places. This can be inconvenient when manipulating with a gizmo.
In this case, you can use Length in Thou wich meant "Thousandth of an inch":
For more complex cases: work with micro or macro objects - suitable overriding of Unit Scale: