Ian,
Probably because you have decent acceleration and therefore don't notice it. Wind down your accel to a minimum and try it
With version R3.043.022 and using G90 G31 the Z axis comes to a halt after first contact without any regard to acceleration settings, winding down the Acceleration has no effect.
If the trip (or as many trips as you like as it's decelerating after the first trip) is/are momentary i.e. bounced as per my original description of a "glancing blow", then the value stored is where the axis stops after decelerating and NOT where the trip occurred.
This is not what I am finding from the tests I have done so far. As mentioned the value retrieved from Get Var (2002) is the first contact point not where the axis finished up.
Could you perhaps describe a test method that you have used to support your re-writing of the value please.
Tweakie.