Error in movements when switching between mm and inch

So I discovered noticed something odd when i was checking my machines movement when i told it to move 5mm through axis using the fixed increment size my machine moves almost exactly 4.999 mm according to a very nice digital indicator and 10.000 mm when jogged another 5mm. Awesome! But when I moved jogged by .5 inches I moved .5016 inches and when I jogged again I measured 1.0028 inches switch back to mm jogs and the machine move the correct amount as specified in the jog window. I repeated these jogs several time to make sure that this wasn't a fluke.

My axies are all the same movement is exact when given mm commands and off when given inch commands. the machine is configure as a mm machine 1605 ballscrews with 200 step per rev steppers at 10x micro stepping which should give me 2000/5 = 400 steps per rev which is what the .ini file shows.

Is this a rounding error or am I missing something? Thanks in advance.

I can force my post to output gcode in mm for now but it would be nice for the machine to not care what units were given.

Could you try your test again but this time use MDI commands to do your inch jogging?
G20 G91;
G0X0.5;
X0.5;
X...
Then try
G20 G90;
G0 X0;
X0.5;
X1.0;
X...
Are the results from both of the above tests the same? Are they closer to correct?

Not sure how you could have a rounding error, 1" would be 2032 steps/rev, and a 1/2" jog should come out to an even 1016.