I have a D25 NC serial number 251513. The distance measurement seems to be stuck at 90mm when reading a sample. I have run the diagnostics >distance Sensor check, and the distance does change if different height samples are presented, however when a normal reading is performed, it always states the distance at 90mm.
In the Global Options for the sensor scroll to the Measure Settings. The choices are Fixed or Auto. Based on your description the setting must be Fixed, with a setting of 90mm. If you change the selection to Auto, distance correction will be applied based on the actual measured distance.