Altitude is the least reliable indication from GPS and really is only given for information only. Unlike your aircraft altimeters which derive their readings from air pressure around your aircraft, and hence are pretty accurate as they are only reading the difference between your position and ground level, GPS altitude readings are derived by working out where all the distance spheres from each satellite are intersecting. Hence you are not measuring the difference between your position and ground level, but the distance between yourself and multiple satellites, which brings with it calculation and position errors.