Why GPS elevation varies

I see a lot of concerns about GPS elevation. Its not the app. GPS is very accurate laterally (X & Y) but very weak vertically (Z). If it was possible to receive a GPS signal from "below" then it would be great vertically too, but not possible. It takes at least three dispersed location sats "circling" your point to get an accurate reading (basic triangulation). Easy for lateral measurement, impossible for vertical. Im amazed it does as good as you say.

Cork puller about Current Elevation