There are two possible reasons for this. I am not sure what sensor they are using, but if it is a pressure sensor, then it will have errors based on changes in the atmospheric pressure. altimeters on aircraft have an adjusting knob to account for this. The control tower will provide the pilot with the local barometric pressure to adjust it.
If it is using GPS, then it is likely the vertical error in the GPS, based on which satellites are being used. GPS altitude error is roughly 75 feet, very close to what you are seeing.