Rangefinder SF20 Lidar min range

I did install the SF20 according to the wiki and set the parameters RNGFND_MIN_CM to 5 and RNGFND_GNDCLEAR to 8cm.
However When the copter is on the ground I get a sonar reading of 22cm… (in MissionPlaner)
So I tried to fix this by setting the RNGFND_MIN_CM and RNGFND_GNDCLEAR to 22cm however I still get the 22cm from the sonar…
Shouldn’t it show 0 cm when on the ground with the above settings?