why is per unit flatness deviation zero?
 

1. small per unit area and sparse point desity
The area covered by the meas points of the plane is approx. 168 X 97 mm. Given that there are only 24 points over this area, it is not surprising that the per unit flatness is 0 until you increase the per unit area to 30 X 30. Increasing to a larger area gives a larger per unit flatness as expected.
If you want to use a small per unit area such as 10 X 10 you will need a much greater point density. If the number of meas points in any per unit area is less than 4, then the per unit flatness for this area is 0 indicating not enough data.