G30 Z-Probe does not return the same distance as probed down

[DouglasPearless]

When the Z-Probe returns after a successfully probe, it returns some 0.4mm less than the original travel so repeated G30's eventually do not return high enough to untrigger the Z-Probe itself.

[DouglasPearless]

I have been patiently debugging this for several days, and I discovered that the number of stepper motor down pulses is not the same as the up pulses.

For example, 3383 down but only 3780 up = 53 less pulses!

I am not sure why, but at least I have an area to look at now.