If your DRO shows 1mm Z movement and your actual movement is 20mm, you have wrong steps per mm so you have to calibrate your axes:

$100 = 42.610 (X-axis travel resolution, step/mm)
$101 = 42.610 (Y-axis travel resolution, step/mm)
$102 = 1060.00 (Z-axis travel resolution, step/mm)

Whatever you have in parameter $102, divide by 20 and write it back to the controller (arduino :)):
$102=newly calculated value.