It stands to reason that not every number can be divided by a whole number and come out to an exact whole number.

It your machine is set up in inches, then usually this is becasue the leadscrew is in inches, so a finite number moves the correct distance. The same applies to a machine set up in metric - where the leadscrew is metric - again a finite number will give a finite answer.

Whe you use one to produce the other , the divisor is 25.4 hardly a simple number, and therefore some rounding up must occur. Mach calculates to 12 places of decimals (depsite what is shownon the DRO's) and the error, as Hood said, is very very slight, and I doubt your machine will cut that accurately anyway - we are way past the micron stage.