Distance telemetry is based on GPS readings, which have typical error +/- 10-20ft. Under poor satellite geometry, resulting in a high DOP, location errors can get as high as 50-100' (rare, but they do happen).
Under normal circumstances, do not expect precision better than a 30' radius around the home point. Most of the time it will be better, many times much better. However, you will eventually fly some time when the sat configuration is not favorable, and be surprised to see an RTH land 25 feet away.
As for the altitude, barometric sensors are subject to error from changing barometric pressure due to weather. Any time there is significant weather activity in the area, pressure changes.
In 20 minutes flying, it isn't abnormal for pressure to change enough in your locale that an error as much as 20-30' can occur. This is the reason for the ultrasonic rangefinders on the bottom of the craft -- when closer than 30', greater accuracy is needed than a barometric altimeter.
As
@msinger points out, however, immediately after powering up before flying, it should tare itself to 0ft, at the current (startup) pressure. If not, something's wrong.