GPS readings: why is altitude inaccurate by 50m, while longitude and latitude are more precise?

I'm using the GeoTracker application to record position and velocity as I exercise along a track. I live on relatively flat ground, only shift up and down no more than5 meters at the very maximum, but while latitude and longitude readings seem quite precise, the program, which uses GPS (and IMHO does not use the pressure sensors / barometer to determine altitude) reports the difference between maximum and minimum altitude along the track to be about 50m. This is quite inaccurate it seems. Why is this?

Thanks.