Leadscrew adjustment Numbers, Meaning?

I have never been able to get a clear answer from anybody what these numbers actually mean? When doing the 3 axis level
Leadscrew adjustments made: 0.017 0.027 0.017, points used 3, deviation before 0.020 after 0.000
the deviation before .02 after 000 I dont quite understand what that is telling me? How did it come up with that .020 for example

The 0.02 is the difference between points before the correction. the 0.000 is the difference after correction.

The "deviation before" is the RMS height error of all the probe points before the correction was made. "deviation after" is the predicted error after the corrections. If the number of probe points is equal to the number of leadscrews then "deviation after" will always be zero.