The answer isn't simple.
How do you got the real altitude of the plane ? Good question.
Currently, the ardupilot use mainly barometer but it is not a 100% reliable. The barometer gives an estimation of the altitude based on the pressure difference between the start point and the current position. The pressure reading is done with the sensors accuracy and many things can disturb it : UV, wind, temperature etc. So the estimation altitude from barometer isn't 100% reliable.
Maybe I am wrong but I think that ardupilot fuse the altitude from all the sensor it has available to have a better estimation.
That means that the barometer reading is fused with the rangefinder reading and GPS alt, and perhaps estimation from IMU (?).
About the rangefinder : what the accuracy at 100m ? And does it got the altitude from ground or from a car or a tree ?
For the GPS reading it is the same thing : you only have an altitude estimation ( There are some black magics involved here so I won't try to explain why)
So the question is how to have a reliable altitude estimation when you don't have a good reference ? It isn't simple as you got no way to know which sensors is giving you the better reading.
So about your question, you are looking two different parameters : the reading from the barometer and the altitude estimation send to the GCS. So the fact that the values aren't the same isn't an error for me as they are different things.
PS: about pity that developper doesn't respond. This is a community work. So people answer when they got time or if they know ... And generally it is always the main developper that are answering. So you can't expect a direct answer on a question post since 1 day. It is free-time unpaid help. So be patient. And try to participate yourself on other subject if you can !